One- and Two-Color Laser Spectroscopy with Photoacoustic and Multiphoton Ionization Detection Citation Moll, David Jerry (1983) One- and Two-Color Laser Spectroscopy with Photoacoustic and Multiphoton Ionization Detection. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/60wm-py84. https://resolver.caltech.edu/CaltechTHESIS:10212019-175021360 Abstract The techniques of pulsed photoacoustic spectroscopy and multiphoton ionization spectroscopy have been used to study several types of weak optical transitions in the ultraviolet or visible spectral regions for gas phase molecules. Both one- and two-color experiments have been performed. The one-color experiments involved the study of high vibrational overtones and spin-forbidden transitions. The two-color experiments demonstrated the application of photoacoustic detection of stimulated emission pumping for populating high vibration levels of the ground electronic state. Two-color experiments were also used to study excited state absorption processes in aniline. Pulsed laser photoacoustic spectra of CD 3 H in the region of the ∇v CH = 5, 6, and 7 CH stretch overtones were obtained. These overtones and nearby combination bands displayed sharp rotational structure, indicating a minimum excited state population relaxation time of 5 x 10 -12 sec. A combination sum analysis was used to generate excited state rotational constants B'. These constants reflect the geometries of the excited states, and therefore provide a sensitive probe of the mixing between various zero order excited states. Photoacoustic detection was used to monitor a stimulated emission pumping process in p-difluorobenzene. Using the à 1 B 2u 5' state as an intermediate state, several vibrational levels of the ground electronic state were populated. The photoacoustic technique is an attractive alternative to other techniques because of its sensitivity, simplicity, and its ability to differentiate between stimulated emission pumping and excited state absorption. Time-resolved excited state absorption studies were performed on aniline using the multiphoton ionization and photoacoustic detection techniques. Signals from both of these techniques were measured as a function of the time delay between two laser pulses of different wavelength. The first pulse excited the S 1 0-0 transition. The second pulse excited either S 1 or a triplet state produced by intersystem crossing, depending on the time delay between pulses. Both ionization and dissociation processes were observed. By varying the conditions of excitation, it appears that a given amount of energy can be selectively channeled into either ionization or dissociation pathways. These results were explained using a simple Franck Condon factor model. The application of pulsed laser photoacoustic spectroscopy to the study of weak absorptions in the visible and ultraviolet spectral regions was investigated. Photoacoustic spectra of triplet states in CS 2 , SO 2 , biacetyl, and thiophosgene were obtained. An attempt to detect the ã 3 B 1u state in benzene, produced instead a two-photon spectrum of several Rydberg states. It was discovered that the sensitivity of the pulsed photoacoustic technique for extremely weak absorptions is limited by the increased probability of multiphoton absorption when high intensities are used. Finally, in two appendices, the detection of the ã 3 A 2 state in CS 2 by the multiphoton ionization technique is presented. The CS 2 study demonstrates the utility of the multiphoton ionization technique for detecting spin-forbidden transitions at high resulution and with great sensitivity. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: (Chemistry and Applied Physics) Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Minor Option: Applied Physics Thesis Availability: Public (worldwide access) Research Advisor(s): Janda, Kenneth C. Thesis Committee: Kuppermann, Aron (chair) McKoy, Basil Vincent Bercaw, John E. Janda, Kenneth C. Defense Date: 16 February 1983 Funders: Funding Agency Grant Number Department of Energy (DOE) DE-AS03-765F00767 Eastman Kodak Company UNSPECIFIED Shell Companies Foundation UNSPECIFIED Caltech UNSPECIFIED Record Number: CaltechTHESIS:10212019-175021360 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:10212019-175021360 DOI: 10.7907/60wm-py84 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 11838 Collection: CaltechTHESIS Deposited By: Mel Ray Deposited On: 22 Oct 2019 18:08 Last Modified: 07 Aug 2025 00:12 Thesis Files Preview PDF - Final Version See Usage Policy. 78MB Repository Staff Only: item control page

ONE- AND TWO-COLOR LASER SPECTROSCOPY WITH PHOTOACOUSTIC AND MULTIPHOTON IONIZATION DETECTION Thesis by David Jerry Moll In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 1983 (Submitted February 16, 1983) -iiACKNOWLEDGMENTS The studies reported in this thesis required the assistance of many people. help. I am happy to acknowledge their I am especially indebted to my research advisor Professor Aron Kuppermann. He provided a scientific at- mosphere which stimulated my creativity while still requiring thoroughness and attention to detail. He provided the moral support of a true friend in several times of crisis. I thank him from the bottom of my heart. Ron Rianda and Garth Parker both deserve my thanks for their help in performing the experiments described in this thesis. Ron was responsible for most of the electronics hardware and software used in data collection. Garth provided a great deal of assistance during the past year in collecting data and interpreting results. I wish him success as he in- herits the resources · of room 013 and begins his own experiments. The experiments described in Paper I involved collaboration with Joe Perry and Professor Ahmed Zewail. I thank them for their contributions to that work. I would like to thank my friends~ Jerry Winniczek, charlie Loerting, and Jack Kaye for the humor, excitement and practical jokes they added to subbasement life. Charlie and Jerry accqrnpanied me on numerous expeditions throughout the southwestern u.s. Jack was always willing to discuss scientific as well as nonscientific problems. -iii Both Professor J. L. Beauchamp and Professor K. C. Janda took an interest in my work. It was partially through their efforts, I am sure, that I was able to obtain permanent employment. The chemistry department instrument and electronics shops played an important role in the construction and maintenance of the equipment used in my experiments. I want to thank Bill Schulke, Tony Stark, Guy nuremberg, Ray Garcia, and Delmer Dill of the instrument shop; and Tom Dunn of the electronics shop. In particular, Tom provided a long-term loan of some equipment that was essential for the completion of the two-color experiments. I am indebted to Heidi Tanciar and Adria Md-fillan for their assistance in dealing with Caltech's bureaucracy. I also must thank Kathy Lewis as well as Heidi for their excellent typing of this thesis manuscript. I am grateful to the Department of Energy, Eastman Kodak Company, the Shell Companies Foundation, and the Caltech Chemistry Department for financial support during my graduate student career. Finally, I am especially thankful to my parents, my wife Sharon, and her parents for their support and encouragement during my stay at Caltech. -ivAbstract The techniques of pulsed photoacoustic spectroscopy and multiphoton ionization spectroscopy have been used to study several types of weak optical transitions in the ultraviolet or visible spectral regions for gas phase molecules. Both one- and two-color experiments have been performed. The one-color experiments involved the study of high vibrational overtones and spin-forbidden transitions. The two-color experiments demonstrated the application of photoacoustic detection of stimulated emission pumping for populating high vibration levels of the ground electronic state. Two-color experiments were also used to study excited state absorption processes in aniline. Pulsed laser photoacoustic spectra of CD 3H in the region of the obtained. ~vCH = 5, 6, and 7 CH stretch overtones were These overtones and nearby combination bands dis- played sharp rotational structure, indicating a minimum 12 excited state population relaxation time of 5 x 10sec. A combination sum analysis was used to generate excited state rotational constants B'. These constants reflect the geometries of the excited states, and therefore provide a sensitive probe of the mixing between various zero order excited states. Photoacoustic detection was used to monitor a stimulated emission pumping process in p-difluorobenzene. Using the -v- A1 B2u 5' state as an intermediate state, several vibrational levels of the ground electronic state were populated. The photoacoustic technique is an attractive alternative to other techniques because of its sensitivity, simplicity, and its ability to differentiate between stimulated emission pumping and excited state absorption. Time-resolved excited state absorption studies were performed on aniline using the multiphoton ionization and photoacoustic detection techniques. Signals from both of these techniques were measured as a function of the time delay between two laser pulses of different wavelength. first pulse excited the s 1 0-0transition. The The second pulse excited either s 1 or a triplet state produced by intersystem crossing, depending on the time delay between pulses. Both ionization and dissociation processes were observed. By varying the conditions of excitation, it appears that a given amount of energy can be selectively channeled into either ionization or dissociation pathways. These results were ex- plained using a simple Franck Condon factor model. The application of pulsed laser photoacoustic spectrescopy to the study of weak absorptions in the visible and ultraviolet spectral regions was investigated. Photoacoustic spectra of triplet states in cs 2 , so 2 , biacetyl, and thiophosgene were obtained. An attempt to detect the a3 B1 u state in benzene, produced instead a two-photon spectrum of several Rydberg states. It was discovered that the sensitivity of -vithe pulsed photoacoustic technique for extremely weak absorptions is limited by the increased probability of multiphoton absorption when high intensities are used. Finally, in two appendices, the detection of the a3A2 state in cs 2 presented. by the multiphoton ionization technique is The cs 2 study demonstrates the utility of the multiphoton ionization technique for detecting spin-forbidden transitions at high resulution and with great sensitivity. -viiTable of Contents .... Acknowledgments. . . . . i i Abstract . . iv Introduction 1 Paper I: High Overtone Photoacoustic Spectroscopy of Some Methyl-Containing Molecules. I. Rotational Analysis • . • • • . . . . . . . . . 12. Paper II: Photoacoustic Detection of Stimulated Emission Pumping in p-Difluorobenzene · · · · 71 Paper III: Two-Color Time-Resolved Photoacoustic and Multiphoton Ionization Spectroscopy of Aniline. Paper IV: . . . . . . . . . . . . . . . . . .103 Detection of Weak Absorptions in the Visible and Ultraviolet by Pulsed Laser Photo~ acoustic Spectroscopy . . . . . • . . . • . • 147 Appendix A: A Description of Laser Systems and Related Electronics . . . . . . . .. 1. Introduction. 2. Laser Systems . .... .186 .187 . . 187 2.1 MY34 Nd:YAG Laser. .187 2.2 DL18 Dye Laser •• .189 2.3 UV400 N2 Laser and DL200 Dye Laser · .192 3. Crystal Phasematching Angle Control . . • . . . 193 4. Electronics for Two-Laser Experiments • Appendix B: The Design of Photoacoustic Cells for Pulsed Laser Excitation • · . .. .. .. 1. Introduction. 2. Photoacoustic Cell Design 2 .1 PACl . . . .. . . . . . . 197 • •2 0 7 . . . . . . 208 .2 0 8 . .208 -viiiTable of Contents (continued) Page 3 •. 2. 2 PAC2 . . 209 2.3 PAC3 . . .211 2. 4 PAC 4 • . .212 2.5 PAC5 . . .214 Conclusions • . 214 Appendix C: Appendix D: Wavelength Conversion by Stimulated Raman Scattering . . • . . . • . • . . . . . 217 Detection of the 3A2 State of cs by Multiphoton Ionization . . . . ~ 2. . . . . • 228 Proposition I: A Direct Measurement of the Internal Conversion Rate in the S State of Naphthalene Using Picose~ond Two-Color Time-Resolved Multiphoton Ionization Spectroscopy • . . . . . . . . . . . . .241 Proposition II: Collisional Relaxation of Rovibra tional Levels of Formaldehyde Prepared by Stimulated Emission Pumping • . 251 Proposition III: An Experimental Investigation of the Mechanism of Plasma Formation d~ring Metal Processing with High Power Ultraviolet Lasers . . . . . . . 260 Proposition IV: H7 0 Vapor Detection in the Chemical OXygen Iodine Laser by a New Dye Laser Intracavity Absorption Technique Proposition V: . . . . . . . . . . . . . . .2 7 3 A Radio-Chemical Tracer Study of the Tungsten Regenerative Cycle in Incandescent Lamps Containing CBrF 3 . . . . . 283 -1- Introduction -2- The experimental work described in this thesis is concerned primarily with the detection of weak optical absorptions in the gas phase. The optical transitions studied in the course of this research were weak for one of the following reasons. (1) The absorption involved a multi-quantum vibra- tional transition in the ground electronic state. (2) The transition resulted from stimulating emission out of a sparsely populated electronic state back to the ground electronic state. (3) The transition involved an excited state absorption process from an electronic state with a small population. (4) The absorption resulted from a spin- forbidden transition between states of different spin multiplicities. The results presented in papers I-IV are spectroscopic studies of transitions from each of the above four categories, respectively. They were obtained using the photoacoustic spectroscopy (PAS) or multiphoton ionization (MPI) detection techniques. These techniques will be described briefly in the next two sections. Photoacoustic Spectroscopy The photoacoustic effect was first described in detail 1 by Alexander Graham Bell in 1881. Recently, due to the widespread availability of lasers, it has been used extensively in studies of gases, as well as liquids and solids. Excellent reviews of this technique can be found in books -3- . h e d by Pao 2 an d Rosencwa1g. · 3 pu bl 1s In a typical gas phase photoacoustic experiment, an intensity-modulated light beam is directed into a chamber containing the gas sample to be studied. If the sample absorbs some of the incident light, the molecules excited by the absorption process may undergo collisional deactivation. Eventually, some of the initially absorbed energy appears as translational energy of the molecules and is detected by a microphone as a periodic pressure increase. Recording the microphone signal as the wavelength of the incident light is scanned produces a photoacoustic spectrum. In the PAS experiments described in this thesis, a tunable pulsed dye laser was used as the excitation source. Molecules excited by a <10 nsec pulse from this laser typically undergo collisional relaxation in a few ~sec. A local pressure increase appears in . the region of the laser beam. This pressure increase propagates radially outward as a cylindrical pressure wave. It is detected when it strikes a microphone mounted in the wall of the photoacoustic cell. A pulsed laser photoacoustic spectrum is generated by monitoring the amplitude of this pressure wave as a function of excitation wavelength. The PAS technique has been used widely in the study of weak absorptions arising from high vibrational overtone 6 7 transitions, 4 ' 5 detection of atmospheric pollutants, ' and -4- and excited state absorption. 8 ' 9 We have used the pulsed PAS technique to investigate both high vibrational overtones and spin-forbidden transitions. Paper I presents the results if a pulsed PAS study of high vibrational overtones of cn H. 3 The resolved rotational structure in these spectra permitted rotational analysis which generated the excited state rotational constants B'. Since these constants reflect the geometry of the excited state, they provide a sensitive probe of the mixing between various zero order vibrational states. Paper IV presents a discussion of the applicability of the pulsed PAS technique for studying extremely weak spin-forbidden transitions to triplet states. Several examples of triplet state spectra obtained using the pulsed PAS tehcnique are given to illustrate the advantages and disadvantages of that technique. A second, completely different class of PAS experiments focuses on the dynamics of excited states in molecules. Since the PAS technique is sensitive to the nonradiative decay of excited molecules, it provides a unique tool for studying excited state dynamics. The wide variety of problems studied in the past includes investigation of vibrational relaxation rates, 10 thermochemical effects following dissociation, 11 electronic relaxation kinetic effects, 12 and radiationless transitions. 11 , 13 All of these studies used a CW laser PAS technique in which the change of phase angle of -5- the PAS signal with respect to the intensity modulation was monitored. This method permits time-resolution studies in the 100 ~sec timescale or slower. We have developed a new gas phase two-color time-resolved pulsed PAS technique which permits a new class of dynamics experiments to be performed with nanosecond or faster time resolution. Briefly, the output beams of two pulsed dye lasers are overlapped inside a photoacoustic cell in a collinear counterpropagating configuration. The time delay 6t between the output pulses of the two lasers can be adjusted with nanosecond precision. The first pulse can excite an electronic state of the sample molecules, which can fluoresce or nonradiatively transfer to another state through internal conversion or intersystem crossing. Depending on the value of 6t chosen, the second pulse will find molecules in the ground state, the state initially excited by the first pulse, or some other state populated by a radiationless process. If the molecules are in an excited state, the second pulse may be absorbed in an excited state absorption process, or stimulate emission out of the excited state down to a lower · state in the molecule . Since the PAS technique responds to the total amount of energy deposited in the sample, the excited state absorption or stimulated emission processes produce larger or smaller PAS signals, respectively. Varying the wavelengths of the laser pulses, or changing 6t, while monitoring the PAS signal allows -6- a new and versatile technique for studying excited state dynamics. Paper II describes the use of the two-color timeresolved PAS technique to populate high vibration levels of the ground electronic state of p-difluorobenzene using stimulated emission pumping. The increase or decrease in the photoacoustic signal provides a simple method for determining whether stimulated emission or excited state absorption has occurred. Paper III presents a study of excited state absorption in aniline performed using the two-color time-resolved PAS technique and a MPI technique. Absorption from both s 1 and a vibrationally excited triplet state produced by intersystem crossing to states lying above the adiabatic ionization potential was studied. It was discovered that a given amount of excitation energy can be selectively channeled into either ionization or dissociation, depending on the way in· which the energy is deposited in aniline. Multiphoton Ionization Spectroscopy When very intense photon fields interact with a molecule multiphoton absorption processes become highly probably. Under these conditions, the molecule may be excited above the ionization potential and subsequently ionize. In typical MPI experiments, an intense UV or visible laser pulse is -7- focused into a gaseous sample, producing an ionization current which is measured as a function of wavelength. When integral multiples of the photon energy are resonant with states in the sample, an increase in ionization current occurs. MPI spectroscopy has primarily been applied to the study of two-photon excited states in molecules such as 14 15 . 16 . 17 benzene, ' 1,3-butadiene, hexatriene, etc. Recent MPI experiments have detected the masses of fragments produced and/or the kinetic energy of released electrons in molecules such as NO, 18,19 r 2 , 18,20 H S, 21 NH 3 , 22 2 25 . . 26 27 18 20 23 24 benzene, ' ' ' acetaldehyde, aniline, azulene, 27 naphthalene and organometallics. 28 Paper III presents a study of aniline which used the MPI technique to complement the results obtained with the two-color time-resolved photoacoustic technique. In the MPI experiments, the wavelengths of the two-laser pulses and 6t were also varied, but an ion current was detected instead of a photoacoustic signal. The combination of both MPI and PAS detection in these experiments provided a powerful method for probing excited state dynamics above the ionization potential. Appendix D contains a paper which describes the u~e of 29 . . . GS . . f or b.I dd en transitions a Mp I tee h nique to d et·ect spinIn · 2• The -3 a A· 2 state of cs 2 was detected using a one-photon spinforbidden transition as a rate limiting initial step. Subsequent absorption of additional phot6ns through the triplet -8- manifold ionized the molecule. Therefore, by measuring the ionization current as a function of laser wavelength, the spectrum of the 3A2 state was generated. This technique promises to allow measurement of spin-forbidden transitions with great sensitivity and high resolution. A description of the laser systems and related electronics used in the PAS and MPI experiments is contained in Appendix A. Appendix B contains a description of the photoacoustic cells used in these experiments and summarizes the important criteria for optimizing cells for pulsed laser excitation. Appendix C contains a discussion of wavelength conversion by stimulated Raman scattering (SRS), and a description of a high pressure cell for SRS in H2 . -9- References 1. A. G. Bell, Philos. Mag. 11, 510 (1881). 2. Y.-H. Pao, edit. Optoacoustic Spectroscopy and Detection (Academic Press, New York, 1979). 3. A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy (Wiley, New York, 1980). 4. K. V. Reddy, D. F. Heller and M. J. Berry, J. Chern. Phys. J..2..., 2814 (1982) . 5. K. K. Lehmann, G. J. Scherer and W. Klemperer, J. Chern. Phys. ~' 6441 (1982). 6. L. B. Kreuzer and C. K. N. Patel, Science 173, 45 (1971). 7. C. K. N. Patel, E. G. Burkhardt and C. A. Lambert, Science 184, 1173 (1974). 8. V. P. Zharov, V. S. Letokhov and E. A. Ryabov, Appl . g, 15 (1977). 9. C. K. N. Patel, R. J . Kerl and E. G. Burkhardt, Phys. Rev. Lett. 10. ~' 1204 (1977). T. L. Cottrell, I. M. Macfarlane, A. W. Read and A. H. Young, Trans. Faraday Soc. 62, 2655 (1966) . 11. W. R. Harshbarger and M. B. Robin, Ace. Chern. Res. ~' 329 (1973). 12. W. R. Harshbarger and M. B. Robin, Chern. Phys. Lett. l!, 462 (1973). 13. T. F. Hunter and M. G. Stock, Chern. Phys. Lett. ~' 368 (1973); J. Chern. Soc. Faraday Trans. II 70, 1028 (1974). -1014. P. M. Johnson, J. Chem. Phys. g, 4562 (1975). 15. P. M. Johnson, J. Chern. Phys. 64, 4143 (1976). 16. P. M. Johnson, J. Chern. Phys. 64, 4638 (1976). 17. D. H. Parker, s. J. Sheng and M. A. El-Sayed, J. Chern. Phys. 65, 5534 (1976). 18. L. Zandee and R. B. Bernstein, J. Chern. Phys. 71, 1359 (1979). 19. J. C. Miller and R. N. Compton, J. Chern. Phys. 75, 22 (1981). 20. J. C. Miller and R. N. Compton, J. Chern. Phys. Zi' 2020 (1981). 21. J. C. Miller, R. N. Compton, T. E. Carney and T. Baer, J. Chern. Phys. 22. ~' 5648 (1982). J. H. Glownia, S. J. Riley, S. D. Colson, J. C. Miller and R. N. Compton, J. Chern. Phys. 23. U. Boesl, H. J. Neusser and E. W. Schlag, Chern. Phys. ~' 24. zz, 68 (1982). 193 (1981). J. T. Meek, R. K. Jones and J. P. Reilly, J. Chern. Phys. 11_, 3503 (1980). 25. G. J. Fisanick, T. s. Eichelberger IV, B. A. Heath and M. B. Robin, J. Chern. Phys. 26. T. G. Dietz, M. A. Duncan, M. G. Liverman and R. E. Smalley, Chern. Phys. Lett. 27. II, 5571 (1980). ~' 246 (1980). D. M. Luhman, R. Naarnan and R. N. Zare, J. Chern. Phys. 7J:.., 3034 (1980). -1128. G. J. Fisanick, A. Gedanken, T. S. Eichelberger IV, N. A. Kuebler and M. B. Robin, J. Chern. Phys. ~' 5215 (1981). 29. R. Rianda, D. J. Moll and A. Kupperrnann, Chern. Phys. Lett • .zl, 469 (1980). -12- Paper I: High Overtone Photoacoustic Spectroscopy of Some Methyl-Containing Molecules. I. Rotational Analysis. -13~igh Oxer!gn~Photo~coustic SEeC~SOEY qf_§gm~ Containing Molecules. I. Methyl- Rotational_Analysi§a) D. J. Moll, b) J. W. Perry, A. Kuppermann, and A. H. Zewail Arthur Amos Noyes Laboratory of Chemical Physics,c) California Institute of Technology, Pasadena, CA 91125, USA (Received Pulsed laser photoacoustic spectra of CD 3 H in the region of the CH stretch overtones sented. ~vCH = 5,6, and 7 are pre- These overtones and nearby combination bands display sharp rotational structure, indicating a minimum excited state population relaxation time of 5xl0- 12 sec. A combin- ation sum analysis is used to generate excited state rotational constants B'. These constants reflect the geometry of the excited state, and therefore provide a sensitive probe of the mixing between various zero order excited states. The nature and extent of this mixing are discussed, and a model for a statistical rotational constant is presented. a) Research supported in part by the U. S. Department of Energy (Contract No. DE-AS03-765F00767, Project Agreement No. DE-AT03-76ER72004). Report Code CALT-76764- b) Work performed in partial fulfillment of the requirements for the Ph. D. degree in Chemistry at the California Institute of Technology. c) Contribution No. -14- I. INTRODUCTION ~ Recent spectroscopic studies of highly excited vibrational states of polyatomic molecules have yielded important information about them. 1 - 13 Many of these studies have attempted to elucidate (1) the nature of such states populated by a single photon absorption, and (2) the dynamics of the intramolecular vibrational energy redistribution which follows their excitation. In most spectroscopic studies of high vibrational states of polyatomic saturated hydrocarbon gases at room temperature, the predominate features appearing in the spectra have been attributed to CH or CD stretch overtones. These features are typically broad (50-150 cm- 1 wide), nearly Lorentzian in shape, and the intensity of the absorption bands decreases as the vibrational quantum number of the overtone increases. The pure overtone bands are sometimes closely associated with weaker nearby combination bands. This broadness and near- Lorentzian shape have been attributed to homogeneous lifetime broadening on the 50-100 femtosecond timescale. 7 Similar studies of low tempeTature crystals of naphthalene 11 , 12 and 13 durene have also yielded Lorentzian lineshapes but with widths as narrow as 25 em-•, Even this broadening severely limits the amount of useful information which can be obtained by spectroscopic means. In addition to the band line shape characteristics, the only other parameters usually obtained in these experiments are the absolute absorption strength - 15- and the transition energy. In this paper (I) and the following one (paper II 14 ) we present spectroscopic results for several high vibrational states of CD3H, and discuss their implications for the nature and dynamics of these states. This molecule has a CH stretch- type oscillation whose frequency is substantially different from any of the molecule's other fundamental frequencies. This fact, coupled with the relatively small moments of inertia of the CD3H symmetric top and the low density of vibrational states results in high vibrational states having features which are sufficiently isolated and sufficiently resolved to give much more detailed information about these states than is usually possible for larger and more complex molecules. In this paper we present an interpretation of the rotational features associated with the v= 5,6 and 7 vibrational overtones of the totally symmetric CH stretching mode (v1= 2992 cm-1) of CD3H and several nearby combination bands. In paper (II) we present an interpretation of the vibrational characteristics of the same vibrational overtone bands of CD3H, and present overtone spectra of several similar mol.,. ecules. -16- 2. Background CD3H is an oblate (Iz>Ix=Iy) symmetric top molecule belong- ing to the C3v point group. the CH CD 3H. ~ Many of the degeneracies present in spherical top rotational spectrum are removed in In particular, parallel transition between levels of A1 symmetry in c3v molecules are not influential by first15 order Coriolis effects, unlike perpendicular bands. Therefore, it should be possible to obtain unique reliable values for rotational constants for these bands. Such excited state rotational constants reflect the overall geometry of these states and provide a sensitive indication of the mixing present between various nearly degenerate excited vibrational levels. The vibrational absorption spectrum of CD 3H was first measured by Benedict, et al. 16. Rotational studies of the v=l and v=2 levels of the v 1 totally symmetric CH stretch modes were 17 18 carried out by Rea and Thompson, and Wiggins, et al. re15 spectively. The v=3 and v=4 levels were studied by Bovey. The highest resolution study of CD 3H was carried out by Jennings and Blass 19 on the Vz fundamental which is a par - allel type band corresponding to the CD3 symmetric stretching mode centered at 2143 em - 1. We believe that the ground state parametersof Jennings and Blass are the most accurate ones to date. A listing of known vibrational and rotational constants is given in table 1. Gray and Robiette 26 have recently -17- determined the quadratic and cubic potential constants of methane by fitting spectroscopic data from six isotopic methanes including CD 3 H. In this paper we present the pulsed laser photoacoustic spectra of the v v1 = 5,6 and 7 vibrational overtones of the totally symmetric C-H stretching mode of CD 3 H, as well as nearby combination bands involving one less quantum of CH stretch and the two quanta of the degenerate CH bending motion of E symmetry. Five of the bands exhibit distinct rotational structureswhich permit a combination sum rotational analysis from which the vibrational transition energies and the excited state rotational constants B' are obtained. The values of B' reflect the overall geometry of the excited state. Since each vibrational state has its own specific geometry and rotational constant, the interaction of different vibrational states produc~deviations from the expected rotational constants. In the case of CD 3 H these deviations are substantial and indicate significant mixing of vibrational states . 3. ExEerimental The spectra presented in this paper were obtained using a pulsed laser photoacoustic spectrometer. A detailed description of this apparatus will be given elsewhere, 27 but some of its characteristics as well as the sample preparation and data handling are summarized below. 3.1 Sample Preparation The CD 3 H sample used in these experiments was obtained from Stohler Isotope Chemicals, and was used without further purification. as 99% D. The isotopic purity of the CD 3 H was specified The photoacoustic cell was evacuated to lxl0- 6 torr for at least 12 hours before filling with CD 3 H. The photo- acoustic spectra in the region of the vaS and 6 CH stretch overtones were obtained using a CD 3 H pressure of 370 torr, while the reduced transition intensity of the v=7 spectrum required a sample pressure of 1385 torr. change over a period of several days. The spectra did not They showed sharp ro- tational structure, which is absent in the corresponding CH~, CDH 3 and CD 2 H2 spectra (see paper II). We conclude that this sharp structure cannot be attributed to isotopic impurities~ small leaks, or outgassing of the photoacoustic cell. 3.2 Laser System The laser system consisted of a Molectron 28MY-34 pulsed Nd:YAG laser and a Molectron DL-18 dye laser. The second and third harmonics of the amplified Nd:YAG output were used to ~ 19- pump dye solutions of Exciton oxazine 725, rhodamine 640, or coumarin 540A in methanol or ethanol. The oxazine 725 os- cillator output was amplified once by a transversely pumped dye amplifier. The rhodamine 640 oscillator output was ampli- fied by a longitudinally pumped dye amplifier. The coumarin 540A oscillator output was amplified by two consecutive transversely pumped dye amplifiers. Individual pulse energies were monitored by measuring the light reflected off two consecutive thin quartz flat beamsplitters with a Laser Precision RKP-335 pyroelectric detector. Laser pulse energies varied as a function of wavelength within the range between 2 and 15 mJ/pulse. Dye laser FWHM pulse widths were reported by the manufacturer to be about 4xlQ- 9 seconds. The pulse repetition rate was 10 Hz. Absolute wavelength calibration was carried out using an iron-neon hollow cathode discharge lamp dispersed by a 0.5 meter Ebert-type monochrometer. The absolute accuracy of the 0 wavelengths reported in this paper is ±0.5 A while the accuracy of the wavelength differences important for determining rota0 tional constants is ±0.2 A. The bandwidth of the laser was -o. 5 em -1 . 3.3 Photoacoustic Cell The photoacoustic cell used in this study was similar to that of West, et a1. 29 Its design reflected the important criteria that (1) the microphone detector was positioned as close as possible to the laser beam to maximize the amplitude .. zo.,. of the outward moving cylindrical pressure wave and (2) scat- ' , tered light striking the walls of the cell near the microphone was minimized to reduce the background signal produced by local heating of the cell walls. The cell was constructed using fittings. 3 /a" brass Swagelock A Knowles Electronics30 BT1759 miniature electret microphone with built-in FET preamp was attached with epoxy resin to the side of a brass 3 /a II Swagelock tee having an in- ner diameter of 9 the cell axis. Carefully cleaned quartz windows were mounted /32 inches such that it was about 6 mm from 6 em away from the microphone and aligned so that both windows were exactly perpendicular to the laser beam. The photoacoustic signal from the miniature electret microphone following the laser excitation pulse has characteristics similar to those described by Chin, et al.31 An initial peak associated with the cylindrical pressure wave reached the microphone approximately 65 ~sec after the laser pulse, followed by a complicated sequence of additional pulses and ringing due to scattered light phenomenon. The delay between the laser pulse and the initial photoacoustic signal varies linearly with the distance of the microphone to the axis of the laser beam and is determined by the speed of sound in the gas. The spectrum is generated by monitoring the amplitude of the first peak as a function of laser wavelength. This amp- litude is a linear function of the energy per laser ptilse. Due to the high intensity associated with pulsed laser beams - 21 - care was taken to minimize nonlinear optical effect . The dye laser beam diameter was nominally 8 mm and an aperture allowed only the central 5 mm of the beam to enter the cell. With this configuration a linear dependence of the photoacoustic signal with pulse energy from 300 torr of neopentane at 633 nm was observed. However : when a contracting telescope was used to reduce the beam diameter to approximately 2 mm the baseline of the neopentane spectrum shifted upwards and became irreproducible. The linear dependence of signal on pulse energy obtained without the telescope and the absence of low lying electronic states in CD 3 H appropriate for inducing resonance enhanced multiphoton electronic excitation or .s timulated Raman processes in this system permits us to conclude that non-linear effects in the measured overtone spectra were unimportant. An additional confirmation of this conclusion is given by the similarity between our pulsed photoacoustic spectrum of CH~ in the region near 619 nm shown in paper(II)and the long path absorption spectrum of Giver 32 and the cw photoacoustic spectrum of Stella, et al.3 3 The effect of sample pressure on the photoacoustic spectra was also investigated, Spectra of neopentane in the re- gion near the v=0~6 CH stretching mode were taken at 102, 300, and 760 torr, the only observed effect being an increase in photoacoustic signal with increasing pressure, without any change in the shape of these spectra. Previous measurements of pressure broadening in the 2v 1 band, the 3v3 band, and a line at -220 6190 A all gave pressure broadening coefficients of approximately 0.087 cm- 1 /atm. 34 - 36 If CD H has similar pressure 3 broadening coefficients the maximum expected pressure broadening in our experiments would be less than 0.2 cm- 1 , well within the bandwidth of our laser. 3.4 Data Handling Electronic signals from the Knowles Electronics BT1759 electret microphone were limited to a frequency range of 300 to 10 4 Hz by an electronic filter and amplified by a factor of about 3xl0 4 • The peak of the first pressure wave was inte- grated by a differential gated integrator system with an integration window of 15 ~sec. Signals from the pyroelectric joulemeter were processed in a similar manner by a second integrator system. A microcomputer system controlled the dye laser grating scan and digitized and stored the signals from both inte- 0 graters every 0.1 A. Each CD 3 H spectrum was scanned several times. The spectra were then laser-intensity-normalized and averaged. The CD 3 H o~s,6, and 7 CH stretch experimental spectra of Figures 1 to 3 respectively represent averages of 24, 36, and 120 laser 0 shots for each 0.1 A. Effective integration time constants and scan rates for the o~s and 6 spectra were adjusted so that the effective resolution was limited to -o.s em-~ by the laser bandwidth. The o~7 transition resolution was limited by the time constant used to -0.7 cm- 1 • -23- The CD3H photoacoustic spectra corresponding to the v= 0 +6 and 7 stretch overtones absorptions displayed large backgrounds which were approximately wave-length independent. We feel that the signal produced by light scattering inside the cell accounts for most of the constant background since the position of the axis of the photoacoustic cell with respect to the laser beam and the cleanliness of the quartz windows greatly affected the magnitude of the background. Due to the uncertainty in the relative contributions of scattered light signals versus a real background absorption we have chosen for purposes of presentation to subtract off 90% of the apparent background in the v= 0 +6 and 7 spectra of figures 2 and 3. The v= 0 -+5 experimental spectrum of figure la was left unchanged. 3.5 Approximate Absorption Cross Sections Approximate absorption cross sections for the various assigned vibrational transitions are listed in table II. The values given correspond to the peak of the central Q branch. These cross sections were estimated by considering the absolute photoacoustic signals, laser energy/pulse, and sample pressures, after calibrating our spectrum of CH 4 shown in fig. 1 with the CHlt absolute absorption cross sections of Giver 3 2 obtained for the some transition by a long path absorption method. The some deactiV-ation efficiencies were assumed in CH 4 and CD 3 H for the purpose of this estimation. -24- The absolute photoacoustic signal exhibits a strong dependence on the positions of the cell with respect to the laser beam, increasing as the microphone moves closer to that beam. When light actually strikes the cell walls, a dramatic increase in signal occurs. For each spectrum measured an at· tempt was made to maximize the true absorption signal and minimize the scattered light signal. The resulting absolute calibration is therefore only approximate. The relative in- tensities of different vibrational states and their implications are discussed in paper rr 14 . A search was also made for state nm to 476.7 nm, but without success. l81) in the region 487.7 Signal to noise con- siderations in that spectral region indicate that state l8z> or any other nearby combination bands must be at least a factor of 2 0 less- intense than transitions to the l61 ,zs) states. 111)- or -25- 4. Rotational Analzsis 4.1 Assignment Figures la, 2, and 3 show pulsed photoacoustic spectra of CD 3 H in the region of the v~O+S,6, and 7 overtones of the tot- ally symmetric CH stretching mode. Each spectrum displays two major vibrational bands, each containing a clearly resolved P and R branch and a sharp central Q branch characteristic of a symmetric top parallel type transition.3 7a The overlap between the two bands becomes stronger for higher overtones, making the v=7 spectrum congested enough that one Q branch is almost ~bscured by the other vibrational band's P branch. The vibrational assignment is discussed in detail in Paper 11, 14 however,a summary of the results is presented below to enable labeling of the spectral features. Various mod- els for vibrational excitation have been used to describe the motion in the excited state, such as the local mode model or the normal coordinate approach. are labeled For clarity the CDsH states with a notation describing the fundamental vibrations in accordance to ref. 26, but we are not implying the nature of the excited states by using this notation. Spec- troscopic studies such as this one combined with theoretical modelS~'~ should significantly advance our understanding of these states. The features ~s assigned in Paper II center~d at approx~ -26 imately 724 . 5 nm, 616.1 nm, and 541.2 nm correspond to the v= o~5 , 6, and 7 transitions respectively of the totally symmetric v1 CH stretch mode, henceforth designated as 151>, l61), and 17~) . The features centered near 731.5 nm, 618.8 nm, and 539.4 nm correspond to transitions from the ground vibrational state to an excited combination band con~ sisting of two quanta of the degenerate e CH bending v 5 mode and 4,5, or 6 quanta respectively of the totally symmetric v~ CH stretch mode . We will designate these states by I4J,2s> I 5~,2s) , and l61, 2s) • An additional feature near 544.7 nm is tentatively assigned as a combination band involving 6 quanta in the v 1 mode, one quantum in the v 5 mode and one quantum in the v s degenerate CD 3 bending mode, denoted by I 61 , ls, ls) . The rotational energy levels of an oblate symmetric top molecule in a specific vibrational level are given by (1) where B, C, n3 , DJK' stants. 37b For a parallel band 6K .,. 0 and 6J = 0, ± 1 except and DK are the usual rotational con- that 6J = 0 is forbidden if K=O . Using these selection rules the wave numbers of the P, Q, and R branches are given respectively by -27- JK "J (J + 1 ) ] K + D 2 - (Za) (D ' - D ") K .. K K (Zb) R(J,K) • (vo + 2B' - 4DJ' + (3B'-B"-12DJ')J + (B'-B"-13DJ' - (B' -B") - DJK' (J+l) (J+Z) + DJK"J(J+l)) K2 - (2~) (D ' - D " ) K .. K K where the prime and double prime refer to excited and ground vibrational states respectively. A computer program utili-zing Eqs. (2a) through (2c) and the appropriate Boltzmann factors, degeneracies, matrix elements, and spin statistics 37 c was written to generate symmetric top rotational spectra. The excellent signal to noise ratio in the spectrum of fig. la suggested using a least squares routine to adjust parameters in the spectral generation program to reproduce the experimentally observed spectrum. This was attempted using the CHD3 ground state parameters of Jennings -28- and 1Has; 9 and a FWHM linewidth of 0.5 cm- 1 • Excited state parameters were adju~ted from the v~4 data of Bove~ until an approximate fit was observed, The least squares routine was then allowed to make final adjustments on the excited state parameters. Values for B' and v 0 for the spectrum of figure laconverged easily. These values along with constants generated by the combination sum approach of section 4.3 are given in TableYIJL The results of the two methods agree within the corresponding error limits. Other excited state parameters did not satisfactorily converge because experimental resolution prevented observation of individual K components, the presence of noise, and the possibility of excited state perturbations. Figure Th shows a typical spectrum generated by the least squares routine corresponding to the region near the I S1 I 41,2 s) states of CD3 H. and It should be noted that the rates of the intensities of these two bands was one of the adjustable parametersin this fitting routine. Similar least ~quares fits were attempted for the spectra in figures 2 and 3. background fits and a (~ee The increased noise and large constant section 3.4) limited the accuracy of these combination sum approach yielded more reliable results (see section 4.3). The synthesized spectra were used, however, to assign the individual J transitions in the congested regions of overlapping rotational features. The resolution of the laser system (0.5 cm- 1 ) prevented -29- the observation of single K components of specific J rot ational transitions, although shading of individual J lines is observed at high values of J. Comparison of synthesized spectra at our experimental resolution with actual positions of the K=O lines indicated that the maximum of each J transition occurred at the position of the K=O line. Tables 3-7 contain the position of the peaks of the various J tran- sitions, as determined from the digitized and averaged data . These positions correspond roughly to the positions of the K=O transitions. 4.2 Ground State Combination Differences Ground vibrational state rotational constants can be accurately determ~ed even in the presence of excited state perturbations using the technique of ground state combination differences.25,40 The combination difference l!F"(J,K) = R(J-1) - P(J+l,K) involves transitions having the same upper state. t~F"(J,K) From equation (1) = F(J+l,K) - F(J-l,K) = (4B"-6D J "-4D JK "K 2 ) (J+ !2) (S) rur photoacoustic spectra yield only approximate positions of the K=O lines and D/' is much smaller than B" . Therefore a plot of [R(J-1) - P(J+l) J/CJ+!2) vs. (J+~) 2 should yield a line with slope 8D 3 " and intercept 4B". B" was determined from spectra of the I 51), I 61),l 41 ,2s) ,I 51 ,2s) , and I 61 ,2s) states of CD3H. The average value of B" obtained in this way was -303.29 ± 0.03 cm- 1 • The relatively large standard deviation is a result of the low resolution which_prevents observation of individual K lines. Our value of B" is consistent with the accurate value of 3.279053 cm· 1 of Jennings and Blass . 19 4.3 Excited State Combination Sums The rotational constants of the upper vibrational state can be obtained by the combination sum R(J-l,K) + P(J,K). 15 , 37 d From equation (2) and (4) R(J-l,K) + P(J,K) = 2vo + 2( (C 1 -B 1 ) - (C"-B")] K2 - 2(DK 1 -DK")K'+ + 2( (B 1 -B") -2 (D J I 2 1 CDJK -D JK ")K -D ")J 2 (J 2 +1) (6) J for K=O eq . (5) reduces to (7) Figures 4-6 + sho:w plots of the combination sums R(J-1 ., 0) P(J,O) vs. J 2 for the states I S1) , l61) , 141) , I S1 ,2s) , and I 61,2~. The lack of curvature indicates that the co efficient 2(DJ 1 -DJ") is much smaller than 2(B 1 -B"), which agrees with values obtained for I 31> and I 41) by Bovey. 15 Least squares fits of our data were used to generate the values of v a and B1 shown in table 8 . The value of B" used was the accurate value 3.279053 cm- 1 of Jennings and Blass . 19 Error limits were determined from standard deviation of the slope and intercept. No strong perturbations are apparent in the $pectra ] -31shown in figures 1 or 2, but this is almost certainly due to the low resolution and congestion in our spectra. Small perturbations in the I 31> and 141) states of CD 3 H were observed by Bovey. 15 The increased density of vibrational states at the energies of these spectra makes the presence of perturbations likely. A rotational analysis of the I 7 1) state and the feature near 544.7 nm was not attempted. The low signal to noise and congestion found in the spectrum of figure 3 made this impossible. An approximate value of B' for the I 61 ,2~ state is listed in tableVIII, but is subject to some question. Intensity and position perturbations for the P(9), P(lO), R(7), R(lOl, R(ll), and R(l2) transitions to the 16 1 ,2s) state, The extreme congestion between the I 11) and I 61,2 s) states and the low signal to noise ratio makes the assignment of the individual J transitions in l61,2s> difficult. The slope of the combination sum line shown in figure 10 definitely is positive, however, indicating that B' for I61,Zs) is larger than B". 4.4 Rotational Temperature The individual J transition intensities in figure Za and 3 are consistent with those generated by the computer program described in section 4.1 if Boltzmann statistics at room temperature are used. The fact that the gas temperature remains at room temperature during the experiment is consistent with the small absorption coefficient (see section 5.2) of the transitions observed here. -32- 5. Discussion CD 3 H displays a CH stretch type oscillation whose fre- ~ency is substantially different from any of its other fund- amental oscillation frequencies. The small number of vibra- tional modes, small interaction between modes, and small moments of inertia have allowed a detailed analysis of the rotational features of several high vibrational states. Unlike benzene where even the v=2 and 3 levels of the CH stretch mode are too broad and congested for rotational analysis, 1 CHD 3 possesses a full range of vibrational overtones of the CH stretch mode plus nearby combination bands which exhibit sharp rotational structure. The excited state rotational constant B' is easy to obtain and is an extremely sensitive indicator of geometry changes in the excited state caused by interaction of both observed and unobserved vibrational states with the state being studied. Therefore the rotation- al analysis provides a sensitive probe of the nature of highly excited vibrational states and the dynamics of intramolecular vibrational energy redistribution following excitation of these states. 5.1 Lifetimes The low density of vibrational states present in CD3H and the large difference in frequency between the CH stretch mode and other modes of the molecule leads to the prediction that energy deposited into the CH stretch mode by single photon .-33- absorption is likely to remain localized for a relatively long period of time. This indeed seems to be the case. The R(2), R(3) and R(4) rotational lines of the I S1) state have linewidths of 1.0 cm·l. FWHM The same set of rotational lines for state ( 61) have FWHM linewidths of 1.3 cm- 1 • These linewidths correspond to minimum population decay times of SxlO-l 2 and 4xlO-l 2 seconds respectively.4~43rhese lifetimes represent only minimum lifetime because (1) we are approaching the limit of our laser resolution and (2) we are not observing a single rotational line, but rather a small group of unresolved K lines. Stella, et a1. 33 observed an unassigned rotational line 0 in CH4 at 6457.19 A in the vicinity of the v=6 CH stretch overtone with a linewidth less than or equal to their experimental 0 resolution of 0.03 A, corresponding to a minimum population lifetime of -70xl0 - 1 2. sec. If the lifetime of CH stretch- type made. in C~H is greater than or equal to that of similar modes in CH 4 , the energy deposited into a mode of predominately CH stretch character in CD 3 H by single photon absorption may remain localized in such modes for time scales comparable to the average time between collisions for pressures ~1000 torr. Therefore, CH stretch type modes in CD3H or other similar small molecules may be good candidates for studying the effects of selective high vibrational excitation on bimolecular reaction rates. Of course, the state specific reactive process must in addition be able to compete with deactivation processes - 34- for such studies to be possible. These studies also suggest higher resolution experiments to investigate the linewidth of the R (0) line in states I 41), 1 s1) ,r61) , and r 71) to determine the dependence of the line- width of a single isolated X line on vibrational state. 5.2 Discussion of Rotational Constants Three attempts to explain the CD3H rotational constants obtained in this work are presented in the following three subsections. Only the third is able to qualitatively explain the observed data. In trying to explain the CD3H rotational constants, we considered in succession interpretations based on a) non-interacting vibrational states, b) two interacting vibrational states, and c) three or more interacting vibrational states. Although only the las.t wa,s able to explain the data, infor- mation derived from the preceding two approaches was used in it. We will therefore describe all three. al Non_-JnteJ.:§!Ctj.n_g, §tate mo£el. Figure 7 shows the know rotational constants from this work and previous work for vibrational states of CD 3 H of the form I v1) andlv1 ,zs) . The rotational constant B' corresponds to the moment of inertia along an axis perpendicular to the c3v symmetry axis. 0 and 4, B' v For transitions to states l_'v 1) with V1 between is a linear function of v1 in agreement with the -35- standard expression 37a (8) B[v.] is the rotational constant for the state designated 1 by the set of vibrational quantum numbers [vi], Be is the equilibrium rotational constant, the sum is over all vibrationa! modes, di is the degeneracy of the ith vibrational mode, and the a~ are small constants, The experimental value of a~ is 0.0160 cm- 1 .15 Non-interacting states of the form I v 1) should fall on this line. BS1 exhibits a slight positive deviation from this line and B6 l is shifted substantially above the a~= 0,160 cm- 1 line. One explanation for this behavior is the presence of additional terms in equation (8) of the form s.B1): (v.1 + d. 1 z ) (v.J d. + _1) (9) z As the molecule approaches dissociation, however, vibrational coordinates lengthen rapidly and the B[v Jcurve should bend 1 downward implying that S~j should be positive. The points for B51 and B are above the straight line, implying that 61 B Sij<O. In addition, the size of the deviation for B6 l cannot be accounted for by a small higher order correction. We conclude that the non-interacting states model does not explain the observed rotational constants. -36- bl Two interactin~ states. A more plausible cause is the Fermi resonance between different zero order vibrational states. figures 1 and 2 each show two states of the fo;nn I Y1) and I' (v-l)l,Zs) • As these states get closer together (figure 2) their intensities become comparable, a direct result of Fermi resonance between the two states. ahe details of this Fermi resonance are discussed in paper II.) For two interacting vibrational levels with zero order rotational constants Bn 0 and B.1 0 which . interact with Fermi mixing coefficients a and b the new rotational constants are given by 3 7e,44 (10) where a 2 + b2 = 1 (11) The two new constants Bn and Bi are therefore intermediate between the zero order constants Bn° and Bi 0 • The size of the shift from the zero order rotational constants to the new rotational constants reflects the amount of interaction between the vibrational levels. In CD 3 H the CH bond is perpendicular to the axis corresponding to the B' constant. This explains why the anharmonicity of the CH stretch mode is so effective in increasing the moment of inertia IB and therefore in decreasing B'. On the other hand, an increase in the CH bending mode (v 5 ) vibration quantum num- ..-37- ber decreases the average distance between the H atom and the B axis, causing B' to increase. These qualitative consider- ations suggest that mixing in CH bend character from combination bands of the form I (v-1)_11 2s) with pure CH stretch overtones of the form fv 1 ) effectively increases B' fDom its vl zero order ~~ line, in agreement with our observation. The decrease in rotational constant between the states also agrees qualitatively with the model, where the I 61) I 4.l,2 s) and I 5 1 , 2 s) 2~level Fermi mixing f SJ,2s) state interacts strongly with the one. This simple two-level Fermi mixing model, however, does not quantitatively account for the rotational constants of the combination band states of t~~- f~!:m l '{,1 1 2 5) . A~l~n and Plyler 24 have partially resolved the rotational structure of the A1 They report a value for B' -B" component of the state I 2s) of about 0.01 cm- 1 . Using the Jennings and Blass 19 value for = 3.289 cm- 1 for that state. This value 25 agrees closely with the result B' =- 3.28831 cm- 1 predicted 25 from Dupre-Maquaire, et al's2 3 measurement of a~= -4.627 x B" we obtain B' 10- 3 cm- 1 and Jenningsand Blass'sl9 value of B" = 3.279053 cm- 1 • The rotational constant Bv 25 , in going from B01 , 25 to B41 , 25 or B51 , 25 should only change by the~~ term in equation (8). The dashed line in figure 8 indicates this behavior which 1 , parallels the a~ line of the states fv1). The rotational constants B4J 25 , B51 25 , and B6 fall above ' , this dashed line. If B25 is correct and only a two-level -38- Fermi interaction occurred, all values for Bv and B v ~' 2 5 should have fallen on the dashed line or between it and the l solid line, at least until higher order corrections like Eq. (9) become important. lt immediately follows from Eqs. (10) and (11) that Bn + B.1 = Bn 0 + B.1 0 (12) Eq. (12) is the so~called "sum rule". Using this rule, ° These zero order values, B4° 2 our values for Bn and Bi, and the a~1 line, values for B4 1 , 2 5 0 can be obtained. and BS1,25' and B 0 S1,25' 1, 5 and Allen and Phyler's value for B25 do not fit an equation of the form of Eq. (8), further illustrating the inadequacy of the two level Fermi resonance model for rotational constants to explain the data. c) Three pr ~ore interactina states. To obtain states with rotational constants above the dashed line of figure 7 it is necessary to mix in states with high rotational constant character. These additional states, which we have not observed, must consist of a few states from the bath of background states. Figure 8 depicts three classes of zero order states which we feel must interact to produce the states observed spectroscopically. The optical states are those states which couple optically to the ground state. This coupling may be strong or weak,. Lut it provides t·he optical transition probability necessary for optical copy. . The tv1) states fall into this category. spectres~ The strong -3 9- coupling states are those which would not be seen optically unless they couple strongly to the optical states, borrowing enough optically allowed character to actually show up rectly in optical spectra, category. di~ The !'" V:l, 2 s) states fall into this The weak coupling states are those background states which do not produce spectroscopic features of their own. A small number of these states, however, may couple weakly to both the optical and the strong coupling states. This coupling produces small changes in the spectroscopic parameters of the observed states. Each weak coupling state which couples re~ sults in only small changes in these parameters, but the presence of several such states could amplify this effect to produce substantial deviations from the expected zero order values. The rotational constant B' of CD3H is highly sensitive to the contributions .of the weak coupling states. The two level Fermi resonance model involving states of the form I v1) and I (v1 -1) 1, 2 5 ) does not adequately account for the observed rotational constants. Added contributions of weak coupling states must be invoked to explain qualitatively the observed rotational features. The information in Table 1 is useful in indicating which types of states from the weak coupling states category are likely candidates fo·r introducing higher rotational constant character into the observed spectra. In CD 3 K, as energy is - 40 added into either the v1, Vz or v~ mode, the corresponding B rotational constants decrease. Vs In contrast , the one for increases with increasing excitation energy (a B5 > 0) . behavior for V3 and VG is not known. The As a result, vibration- al states in the region of our spectra which contain substan tial amounts of vs character are likely candidates for coup lings which result in an increase of B. The rotational constant for state I 6 1 ,2 s) has large error limits but seems to also fall substantially above the dashed line in figure 7 . This again implies a need for coup- ling to states of higher rotational constants, such as sev eral weak coupling states or possibly the state appearing at 544.7 run. The rotational features of the spectra presented in this work have been explained qualitatively by introducing coupling between 3 or more zero order vibrational states. An interesting question is: what is the expected behavior of the rotational constants for the higher energy vibrational overtones not yet observed? for ~mall e~c~t~tion energies the rovibrational states of small molecules can be accurately described by simple quantum mechanical models. Each vibra- tional level has a characteristic rotational constant which is inversely proportional to the moment of inertia of that particular state .. At high energies, however, the increased density of states, increased coupling between states, and ~ 41- finite linewidths of individual rovibrational levels produces a behavior which although accurately described by quantum mechanics, is ha·r d to elicit theoretically bacause the corresponding Schrodinger equation is difficult to solve. Under these conditions, however, it is expected that the internal motion of the molecule will approach a classical mechanical behavior. Concurrently, the molecule's motion is no longer limited to specific vibrational modes because of the rapid scrambling of energy between them. In such a high energy classical and statistical limit, the energy and angular momentum are the only easily discernible good constants of the motion. The classical analog of the quantum mechanical rotational constant can be defined in this limit as a statistical rotational constant 45 which is proportional to a time average of the reciprocal of the moment of inertia, averaged over the molecular motions. rn OUT CDJH spectra the rotational constants for different vibrational levels are significantly different from each other, reflect:tng the dominant contributions of various zero~order states. This behavior is characteristic of the quantum nature of the system~ As the classical statistical limit is approached at higher energies, however, the increased scrambling of energy could cause the spectroscopic rotational constants to converge to a statistical rotational constant which becomes independent of the way in which the energy is initially deposited in the molecule and ts a function only -42- of the energy itself. One approach for calculating the statistical rotational constant is to use a classical mechanical model for the rovibrational motions of the molecular motions of the molecular nuclei, and calculate <Bclass ~by calculating the average of the quantity over a trajectory corresponding to total internal energy E, where t is time, mi is the mass of the ith atom, and qi its distance from the moment of inertia axis being considered. In the statist.i cal limit, this average should become independent of the trajectory's initial conditions. Given a potential energy function V describing the interaction between the atoms, the implementation of a classical trajectory calculation for a molecule of the complexity of CD 3 H is feasible and relatively straightforward. 4 6 Numerical studies of this type may be appropriate at the present time~ Comparison of calculated statistical rotational con- stants and observed spe~troscopic rotational constants for vibrational overtones at higher energies may provide a sensitive method for detecting the onset of statistical behavior in CDJH or other molecules, -43- 6. Cone lus ions The exact description of highly excited V-ibrational states in polyatomic molecules has been a subject of con siderable interest. By carefully selecting CD1H, a small molecule with high symmetry, symmetric top rotational structure, small moments of inertia, and low density of vibrational states we have been able to easily obtain pulsed laser photoacoustic spectra of several high vibrational overtones and combination bands which exhibit sharp rotational structure. The analysis of this rotational structure leads to the follow ing conclusions. 1) For CD3H (and presumably other small molecules with resolvable rotational structure) simple rotational analyses of high vibrational overtone spectra provide sensitive methods for obtaining information about the types of zero order vibrational modes which contribute to the actual states excited. 2) The observed high vibrational overtones of the totally symmetric CH stretch in CD3H contain significant contri butions from other types of modes, including degenerate CH bend motions. 3) It is necessary to invoke coupling with background vibrational overtones and combination bands in CD3H. -444} A simple clas$ical t ·: raj ectory model for a statistical rotattonal constant was proposed. Comparisons between calculated statistical rotational constants and spec~ troscopic rotational constants obtained from future studies of higher vibrational overtones may provide a sensitive ~ethod for detecting the approach to statistical behavior in CD3H or other molecules. The wealth of information about high vibrational states in CD 3 H obtained form this low resolution study suggests that the application of higher resolution techniques to CD3H and other similar small ·molecules may provide a detailed quanti~ tative description of the exact nature of their high vibrational states. ~cknowled~ements We gratefully thank Professor K. Janda and Professor R. Marcus for useful discussion, Dr, J. J. Barrett for help with the design of the photoacoustic cell, and Professor J, L. :Beauchamp for supplying the CD3H sample. 5.010(19) x10- 5 -4. 030(69) xlO- 5 D .. b D .. b JK 1.020(59) x10 H .. b,d 1237. 1292.499 2 (8) 114,-l~f ll s)g 1988.87 (3) 1035. Jc 1 oo2. It 3) c lt 6) c 12 v Statee - B 4.64l(ll) xl0- 3 2.062 8(l 2 ) xl0- 2 C'-C" - l. 29 (3 ) xlo- " DJ ' -DJ " Excited State Parameter s 5.15(40) x10 H .. b,d KJ -3.80(15) x10 HJK .. b ,d J - B 1.8 ( 3 ) X 1 0 - 5 K D .. c B' -13" 2.6297(3) C"c - B 3.27 9 053(61) B"b J Va 1 u e (em -I ) Ground State Parameters CD 3H Spectroscopic Parameters from Previous Studies.a Parameter TABLE I. - 2 . 52 ( 8) x l0- 6 DJK'-DJK" 3.50(6) xl0- 6 DK' - DK" ... V1 - 2142.5776 2250. 88(10) 2332. 2564.6 2592.64 2992.33 ltz)b I h)c lts,1~f l25) (A)h l2s) (E)h 1 3181 . ltz.t.)f 11266.94 8623 . 31 13~ 1 1 5865.02 12a)k -.0641 -.0499 -.0381 5135.04 (or 5134 . 84) 11.,12) j -0.0179 0. 007 0.01 -0.059(5) 3456. 14J C' -C" -0.0350 -0.0181 0.0449 -0.023(5) 6.xl0- 6 -1.x10~ 5 -5 -5.0(8)xl0 5.xl0~ 6 -3.x10-5 -11.0(3)x10- 5 7.1(5)x10- 5 2.3(2)x10-5 DK I -DK" 7. 576(54)x10- 6 -1.0045(64)x10- 5 DJK' -DJK" 9.03(43)xl0- 7 Dj'-DJ" Excited State Parameters -1.7549(19)xl0- 2 -2 . 22(3)x10- 2 n•- B" 112. h) f 3327. I t..,h)f 3283. I h,ts,1~t33t5. 3051. I h,2,)f lli) v (Continued) Statee TABLE I. 1 0\ .j:::> Some correspond to 95\ confidence limits and others are standard deviations. are the estimated errors in the last digit of the parameter Please see the references indicated for details. values. Numbers in parentheses (Continued) They are defined in ref . 21. Ref. 18. 1Ref. 15. )c jRef. 25. gRef. 23. hRef. 24. i Ref. 17. f Ref. 22. eThe notation is described in the text, section 4.1. of eq . 1 . dThese constants correspond to small higher order corrections to the energy expression cRef. 20. b Ref. 19. 8 TABLE I. I -.J ~ -48- TABLE II. Estimated Absorption Cross Sections for CD 3 Ha Photon Energy Absorption Cross Vibrational Bandb (em- 1) c Section (cm 2 ) I s1) 13799 (3±l)xl0- 24 I 61) I 11) 16228 (3±l)xl0- 24 18473 (7±3)xl0- 27 l41,2s) 13668 (l±O.S)xl0- 24 I s1 ,2~ I 61 ,2s) 16157 (3±l)xl0- 25 18534 (l±O.S)xl0- 26 18353 aSee section 5.2 for a description of the estimation method. bAssignment is from ref. 14. The notation is discussed in the text, section 4.1. cThe photon energy of the cross section measurement is expressed in vacuum wavenumbers, and corresponds to the peak of the Q-branch of the specified vibrational band. dT entat1ve . . ass1gnment. -49- TABLE I I I. Observed Wavenumbers (vvac) for the I 41 ,Zs) State. a P (J) (em- 1 ) R (J) (em - 1 ) 1 13661. 8 b 13680.3 2 13665.5 13687.6 3 13648.2 13693.6 4 13641.9 13699.7 5 13635.7 13706.0 6 13628.6 13713.5b 7 13621.8b 13719.1 8 13614.8 13725.1 9 13608.2 13731.1 10 13601.0 13737 . 3 11 13593.8 13742 . 9 12 13587.0 13748.7 13 13580.1 13755 . 0 i4 13573.3 13759.5b J 0 13766.6b 15 aThe reported values are accurate to ±0.6 cm-1, and the differences are accurate to ±0.4 em ~ ~. bThese points were not included in the combination sum analysis bacause of incomplete data or uncertainty in the position of the band. -50- TABLE IV. Observed Wavenumbers (vvac) for the ls 1) State . a J 0 13807.4b 1 13813.9 2 13787.6 13819.9 3 13780.6 13825.8 4 13773.6 12831.7 5 13766.6 13837.2 6 13759.5 13842.9 7 13752.3 13848.1 8 13744.8 13853.2 9 13737.3 13858.2 10 13729.8 13863.0 11 13721.5 13867.8 12 13713.5 13872.5 13 13705.3 13876.5 14 13696 . 7 13881.0 15 13688.1 13885.0 16 13679.4 13889.2b 17 13892.8b 18 13896.5b 13651. 4b 19 a- ~ lhe reported values are accurate to ±0.6 em -1 , and the differences are accurate to ±0 , 4 cm- 1 bThese points were not included in the combination sum analysis because of incomplete data or uncertainty in the position of the band. -51TABLE v. Observed Wavenumbers (vvac) for the I 51 , 2 s) State. a P (J) (em- 1 ) R (J) (em- 1 ) 1 16150.9b 16169.4 2 16143.0 16174.9b 3 16136.3b 16182.0 4 16129.5 16189.0 5 16122.7 16194.5 6 16116.0 16200.8 7 16109.0 16206.9 8 16102.2 16212.9 9 16094.9 16217.4 10 16088.2 11 16079.9b 12 16072.7b 16234.Sb 13 16067.8b 16239.3b J 0 ~he reported values are accurate to ±0.8 cm- 1 , and the differences are accurate to ±0.5 cm- 1 • bThese points were not included in the combination sum analysis of incomplete data or uncertainty in the position of the band. -52TABLE VI. J Observed Wavenumbers CV"vac) for the I 61) State. a P(J) (cm- 1 ) R(J) (cm- 1 ) 0 16236.6b 1 16243.0b 2 16216.3b 16249.3 3 16209.8 16255.4 4 16202 . 9 16261.7 5 16195.9 16267.8 6 16189.0 16273.6 7 16182.0 16279.2 8 16174.9 16284.8 9 16167.8 16290.3 10 16160.0 16295.4 11 16152.7 16301.0 12 16145.1 16306.3 13 16137.0 16311.6 14 16129.5 16316.7 15 16121.7 16321.2 16 16113.9 16327.8 17 16104.0 18 16099.4b 19 16092.lb 20 16084.6b ~he reported values are accurate to t0.08 cm- 1 , and the differences are accurate to~ . 05 cm- 1 • bThese points were not included in the combination sum analysis because of incomplete data or uncertainty in the position of the band. - 53- TABLE VII . Observed Wavenumbers (vvac) for the l61,2s) State. a J 0 1 18524.9b 18545.1 2 18518.8 18552.3 3 18512.0 18558.8 4 18505.5 18566.0 5 18499.4 18572.5 6 18492 . 9 18579 . 1 7 18486 . 8 18586.3c 8 18481. ob 18592.1b 9 18473.5c 18598.3c 10 18468.7c 18606.6c 11 18611.lc 12 18618.0c aThe reported values are accurate to ±1 cm- 1 , and the differences are accurate to ±0.7 cm- 1 • bThese points were not used in the combination sum analysis because of incomplete data or uncertainty in the position of the band . cThese points were not used in the combination sum analysis because they appeared to be strongly perturbed. -54TABLE VI I I. CD3H Spectroscopic Constants from Present Studya Assignmentb vo(cm- 1)c,d B, (em- 1 ) e , d I 4 1 , 2 s) 13668.1(1) 3.258(1) 13667.8(9)£ 3.26(2)£ 13801.1 (1) 3.2060(9) 13801.0(2)£ 3.203(3)£ ls1 ,2s) 16156.4(2) 3.247(3) l61) 16230.07(9) 3.202(6) \ 6 1 , 1 5' 1 6) g 18353.0(5)h 171) 18473.0(5)h 16 1 , 2 s) 18531. 9(2) \51> 3.301(7) aObtained by combination sum analysis unless otherwise stated. (See footnote f.) bA ss1gnment . . f rom re f . 14· . 1s The notation is discussed in the text, section 4.1. cVibrational frequencies in vacuum wavenumbers. dValues in parenthesis are the uncertainties in the last digit, equal to 2 standard deviation units. eB, is the rotational constant in vacuum wavenumbers. £Values obtained from least squares fitting of the experimental spectrum. gAssignment is tentative. hinsufficient structure was available for a rotational analysis to be performed, The value given corresponds to the position of the peak of the central Q-branch. -55_F.EFE~ENCES 1. K. V. Reddy, D. F. Heller, and M. J. Berry, J. Chern . Phys. ~ 2814 (1982). 2. G. A. West, R. P. Mariella, Jr., J . A. Pete, W. B. Hammond, and D. F. Heller, J. Chern. Phys. ~' 2006 (1981). 3. B. R. Henry and W. R. A. Greenlay, J. Chem. Phys. 1.1:, 5516 (1980). 4. H. L. Fang and R. L. Swofford, J. Chern. Phys. ~ 2607 (1980). 5. M. S. Burberry and A. C. Albrecht, J. Chern. Phys. ~ 4631 (1979). 6. R. G. Bray and N.J. Berry. J. Chem Phys. _u., 4909 (1979). 7. (a) D. F. Heller and S. Mukamel, in Picosecond Phenomena, edit. by C. V. Shank, E. P. Ippeu and S, L. Shapiro, (Springer, NewYork, 1978), p. 51. (b) J. Chern. Phys. 1L 463 (1979). 8. K. V. Reddy and · M. J. Berry, Faraday Disc. Chern. Soc. &Z, 188 (1979). 9. W. R. A. Greenlay and B. R. Henry, J. Chern. Phys. ~ 82 (1978). 10. B. R. Henry, Accounts of Chern. Res. 1..2.., 207 (1977). 11. J. W. Perry and A. H. Zewai1, J. Chern. Phys. ~ 582, (1979). 12, J. W, :Perry and A. H~ Zewai1, Chem. Phys. Lett,~ (1979), 13, J. W. Perry and A. H. Zewai1, J, :Phys. Chern. ll., 934 (1981). 31 -56- 14. J. W. Perry, D. J. Moll, A. H. Zewail, and A. Kuppermann, forthcoming. 15. L. F. H. Bovey, J. Chern. Phys. GJ, 830 (1953). 16. W. S. Benedict, K. Morikawa, R. B. Barnes, and H. S. Taylor, J. Chem. Phys. 17 . ~' 1 (1937) . D. G. Rea and H. W. Thompson, Trans. Faraday Soc.~ 1304 (1956). lB . T. A. Wiggins, E. R, Shull, J. M. Bennett, and D. H. Rank, J. Chern. Phys. ~ 1940 (195 ). 19. D. E. Jennings and W, E. Blass, J. Molecular Spect. SS, 445 (1975). 20. H. W. Kattenberg and s. Brodersen, J. Mol. Spect. ~ 126 (1976). 21. T. L. Barnett and T. H. Edwards, J. Molecular Spect. ~' 347 (1966). 22. J. K. Wi lmshur$t and H. J. Bernstein, Can. J. Chern . .].i, 2 2 6 (1957) . 23. J. Dupre-Maquaire, J, Dupre, and G. Tarrago, J. Mol. Spect . ...2.Q., 63 (1981). 24. H. C. Allen, Jr. E. K. Flyer, J. Res. Nat Bur. Stand. Sect. A, 25. ~ 145 (1959). W. F. Blass and T. H. Edwards, J. Molecular Spectrosc. ll, 16 (1967). 26. D. L. Gray and A. G. Robiette, Molecular Physics J], 1901 (1979). -57- 27. D. J. Moll and A. Kuppermann, forthcoming. 28. Molectrort Corporation, 177 North Wolfe Rd . , Sunnyvale, California 94086. 29. G. A. West, D. R. Siebert, and J. J. Barrett, J. Appl. phy s . ~ 28 23 (1 98 0 )_ • 30. Knowles Electronics, lnc ~, 310.0 N. MannheiJn Rd, , Franklin Park, Illinois 60131. 31. S. L. Chin, D. K. Evans, R. D. McAlpine, and W. N. Selander, Appl. 32. Optics~' 65 (;982). L. P. Giver, J. Quant. Spectrosc. Radiat. Transfer, -19, 311 (1978). - 33. G. Stella, J. Gelfand, and W. H. Smith, Chern, Phys. Lett. -39. 146 (1970). . 34. J. Gelfand and W. Hermina, Chern. Phys. Lett . .§.§, 201 (1979). 35. L. Darton and J. S. Margolis, J. Quant, Spectrosc. Radiat. Transfer 13, 969 (1973). 36 . J. S. Margolis, J. Quant. Spectrosc. Radiat. Transfer 11' 69 (1971) . 37. G. Herzberg, Molecular Spectra and Molecular Structure II, Infrared and Raman Spectra of Polyatomic Molecules (D. Van Nostrand Co., Inc., New York, 1945) . (a) p. 400; (b) p • 2 6 ; ( c ) p • 4 21 ; 38. (d ) P • 3 91 ; (e ) P • 378 . L. Halonen and M. S. Child, Molecular Physics 11, 1 (1982). -58- 39. R. J. Hayward and B. R. Henry~ J. Molecular Spectrosc. ~ 58 (197 4) . 40. w. E. Blass and T. H. Edwards, J. Molecular Spectrosc. - 2 4 ' 116 (19 6 7 )_ • 41. Von V. Weisskopf and E, Wigner, Z. Phys. 63, 54 (1930). 42. F. Bloch, Phys. Rev, ~7:~ 460 (_1946)_, 43. I. I. Rabi, N. F. Ramsey, and J. Schwinger, Rev. Mod. Phys. '26, 167 (1954). ~' 44. A. Ade and D. M. Dennison, Phys. Rev. 45. The idea of a classical statistical rotational constant emanated from a discuss ion between some 716 (1933). of the authors and R. A. Marcus, whose contribution is hereby acknow!edged. 46. J. D. Me Donald and R. A, Marcus, J. Chern. Phys, 65, 2180 (1976). -59.fig_ure Ca.E,tions FIG. 1. Spectrum of CD.JH in the region near the C!v=S CH-stretching overtone. (a) Pulsed laser photoacoustic spectrum of a 370 torr sample at a temperature of -25° C; it is an average of two scans, each at 12 laser pulses for 0.01 nm wavelength interval. The signal intensities in this and all other experimental spectra presented in this paper were corrected for the variation of the incident laser light intensity with wavelength. (b) Theoretical fit to the experimental spectrum (see Section 4 . 1). The lowerabscissa gives the air wavelength and the upper abcissa the vacuum wavenumber. The ordinates are in arbitrary units and have been normalized to unity at the top of the f S1) Q branch. They represent signal intensity above the actual experimental baseline without a background subtraction, The symbols P(J), Q, and R(J) cor~ respond to rotational transitions of ~J=-1,0, and +1 respectively where J is the rotational quantum number of the ground state. The symbols 1vi, v ·) represent the quantum numbers of . J the upper state, as described in section 4.1. F l'G. 2, Pulsed laser photoacoustic spectrum of CD 3 H in the region near the Av=6 CH~stretching overtone, The spectrum shown used a sample pTessure o;f 370 torr at -25° C and is an average of 3 scans, each at 12 laser pulses per 0.01 nm wavelength interval. The lowe~ a~cissa gives the air wavelength, and the upperabscissa the vacuum wavenumber. The ordinate origin corresponds the approximately 90% of the observed sig- -60- nal at the right hqnd s.id~ of the $pectrum, This baseline correction is due to the backgTound signal from scattered light in the photoacoustic cell (see Section 3.4), FIG. 3. Pulsed laser photoacoustic spectrum of CD3H in the region near the ts.v=7 CH·stretchihg overtone. The spectrum shown used a sample pressure of 1385 torr at ~zso C and is an average of 4 scans, each at 30 laser pulses per 0.01 nm wavelength interval. Theabscissae and the corrections for background and normalization of the ordinate are as described in the caption of Figure 2. and l' vi,vj) FIG. 4. The symbols P(J), Q, and R(J) are as described in the caption of Figure 1. Combination sum P(J) + R(J-1) plot for CD3H vibra- tions states present in the spectrum of Figure 1. Combina .. tion sums for I s1) are indicated by the open triangle symbols and correspond to th.e ordinate scale on the left. Combina- tion sums for [4.1,2s) are indicated by the open circle symbols and correspond to the ordinate scale on the right, ate values are in vacuum wayenumbers. All ordin- The lines represent least square fits to the data, F lG, 5, Combination sum P P) + R (J·l) plot for CD 3 H vi bra- tional states present in the spectrum of Figure 2. ation sums for 1'6.1 Combin- are indicated by the open triangle symbols and correspond to the ordinate scale on the left. Combination -61- sums fort S1,Za are indicated by the open circle symbols and correspond to the ordinate scale on the right. dinate values are in vacuum wavenumbers. All or- The lines represent least squares fits to the data. Combination sum P(J) + (J-1) plot for CD3H state FIG. 6. I 61,Zs). The points shown by open triangles were used in the least squares fitting of the line for the combination sum analysis. The points shown by open circles correspond to strongly perturbed features in the spectrum of Figure 3 and were not used in the combination sum analysis. The ordinate scale is in vacuum wavenumbers. FIG. 7. CD3H rotational constants B' vs. CH stretch quantum numbers. The open triangle symbols correspond to previous studies (see Table I). to our data, The full circle symbols correspond The error bars represent ±2 standard deviations obtained from the least squares fits of the combination sum analysis. The open square symbols correspond to zero order rotational constants for a model involving Z interacting vibrational states of the form I v 1) and I (v·lh ,Zs.) • The solid line is the result of a least square fit to the points I o) , 111) , I z.J) , 1· 3.J) , and 14-V and represents the effects of the CH-stretch quantum number on the rotational constant B' for noninteracting vibrational states. The dashed line is a similar line obtained for states of the form 1 v1,z~). values for B' are in vacuum wavenumbers. The -62- Three classes of zero order states for CD 3 H vibra~ tional overtones. The optical states are the source of the optical transition strength in their immediate spectral region . The strong coupling states are the direct source of only a small amount of optical transition strength, and produce spec~ tral features because of intensity borrowing from tbe optical states. The weak coupling states produce no spectre~ scopic features of their own, but interact enough with the other classes of states to change the latter's spectroscopic features. -63- ~/cm- 1 13600 1~00 P!IQ) 13700 P!!l) I I I I j11 I i)l I I Q P(19) m 13800 13900 I I IT 11I I ljT I I I I j41'2 ) 5 e:\1!1) Q R(O) R(!l) R(IO)R(I!I) R(IO) R(l!\) P(IO) P!!l) ~~~~~H~~l~ir1~Irjrlr1l~l;j~l~l~l~j~l~li~l¥jnil~li~j~llnll~jlnll~ 15,> 1.0 (a) 0. 1,0 (b) o. 740 7~ 718 736 Figure 1 716 ·u; ~ 0.5 Cll N H c c - 0. ........ >- t-j ~ <( 'Tl ...... OQ s:: ...... i5 ~ 0 ~ ::J >- c 1/) - 1.0 I I 16200 622 P~ 2 9) I I 620 F(! 5 ~ I I 1 I I 618 I I I 51 ,25) 16300 II 161) 614 Q R~q) I I ~~ 5.) I I ~()q) I I ~(II~) II R(l2) I 616 Wavelength /nm F(.5 ~ I51 ,25) .P9°J P(IO) P(5) Q R(5) I I I II I I I I I I I I I I Ill I I 16100 Zlvac /cm- - 612 CD 3 H 161) 610 16400 I I ~ 0\ 'T1 1-' • VI (1) '"i ~ ()q ......... c - cQ) 1 /) :.0 ~ 0 ~ ~ ::> ·c 1/) 0. 546 - I io.5~ - 1.0 544 18400 542 P\5) I 540 R(5) 16,,2~) 538 R!'OI 18600 I II Ill II II I\ g Wavelength/nm I' \~ ll. ;17,> II Ill II II P(IO) 18500 llvoc /cm- - 536 CD 3 H 16,,2~) 18700 534 I I I V1 0\ -66~ 1-w::>;[u-rn~ +(r)d] 0 0 0 0 0 0 w f'(') I{') f'(') f'(') f'(') f'(') N f'(') f'(') 1'N 1'N 1'N ~ 1'N 1'- ~ NO 0 f'(') 0 0 N N 0 0 8 f2 I{') Je 1'N N Figure 4 -:> -67- 8N 0 0 0 0 w L{) N r<> N r<> ~ ~ 1-wJ;[(I- rn~ + (r)d] Figure 5 -68- J 37068°~~~--~--~--~----~----~----~'~0----~ 0 37067 -'E u .......... - .---. 16,,25 > 37066 I 0 + -- 37065 J 0:: J £l... ...__. 37064 20 40 80 Figure 6 100 120 -6 9- 3.32 CD 3 H ROTATIONAL CONSTANTS I 161.25> 3.30 15, ,256 0 ' 3.28 IO>'\. 3.26 -'E 3.24 (.) ......... ' "' 14,,256 I 14 1,2 5> 0 '" ( () "' ' I 15,,2 5> '' 3.20 •16,> ' .,, '' '15> 3.18 3.16 0 2 4 '' '' 6 CH Stretch Vibrational Quantum Number Figure 7 ' 8 -70- 25,000,.-.---------------------. Zero Order States for CD 3 H Vibrational Overtones 20,000 -I?;> TE -161> -151'25> -151> -141,25> 15,000 ~ ~ 01 lo... Q) c -14> I w -- 10,000 -13> I 5,000 0~----------------------------------------~ Weak Strong Optical States Coupling States Figure 8 Coupling States -71- Paper II: Photoacoustic Detection of Stimulated Emission Pumping in p-Difluorobenzene. -72- photoacoustic Detection of fl~i~y~at~d Emission PumEing in ~-Difluor~eenzene,a) D. J, Moll b)~ G, R, Parker, Jr. and A. Kuppermann Arthur Amos Noyes Laboratory of Chemical Physics,c) California fnstitute of Technology, Pasadena, CA 91125, USA (Received Photoacoustic detection has been used to monitor a stimulated emission pumping process in p-difluorobenzene. Using the A 1 B2u 51 state as an intermediate state, several vibrational levels of the ground electronic state were populated. The photoacoustic technique is an attractive alternative to other techniques because of its sensitivity, simplicity, and the ability to differentiate between stimulatedemission pumping and excited state absorption. An example of excited state absorption in aniline is given, a)Research supported in part by the U. S. Department of Energy, (Contract No, DE~AS03-76SF00767, Project Agreement No. DE-AT-03-76ER72004). b)Work performed in partial fulfillment of the requirements for the Ph. D. degree in Chemistry at the California Institute of Technology, clcontribution No, -73- It INTRODUCTION Stimulated emission pumping (SEP) has recently been demonstrated as a powerful technique for selectively preparing large numbers of molecules in specific highly excited rovibrational states in the ground electronic state for diatomic or small polyatomic molecules. 1 - 4 The SEP process produces this selective population transfer in two sequential steps. First a PUMP laser excites molecules from an initial level 1 in the ground electronic state to level 2 in an excited electronic state, A second laser, the DUMP, then stimulates emission out of this excited state to a lower energy state, level 3, resulting in an overall transfer of population from level 1 to level 3. SEP therefore provides an alte~nate route for exciting molecules to level 3 that may be much more efficient in some cases than direct excitation from level 1 to level 3. SEP is especially attractive for producing large popu~ lations in specific high vibrational states of the ground electronic state because it often allo~s the substitution of favorable Franck-Condon factors and easily obtainable tunable visible or UV laser ;for weak lflvl > 1 vibrational transitions and difficult to obtain tunable IR lasers. In formaldehyde, for example, it was estimated that greater than 0,02% of the total thermal population within the PUMP beam was transferred to a specific rotational level in the v4~4 vibrational state by an SEP process. 3 This paper describes the use of the photoacoustic effect - 74- for detecting ~nd monitoring the SEP process, and demon~ strates its utility in a specific molecule, p-difluorobenzene (p-DFB) . By monitoring the total energy remaining in the sample after the PUMP and DUMP pulses using the photoacoustic effect, it is possible to determine if the DUMP pulse stimulated emission out of level 2 or if it excited the molecule from level 2 to a higher energy state. -752L previous SEP Detection Techni~ye~ Two techniques have previously been used to monitor the 1 SEP process, Kittrel, et al. and Reisner, et a1. 3 in their studies of SEP in 1 2 and formaldehyde respectively, monitored the change in fluorescence from level 2 (an excited singlet state)_ as a function of the DUMP pulse wavelength. When the DUMP stimulated emission out of level 2 to level 3, this fluorescence decreased, To minimize the effects of pulse to pulse variations in the PUMP intensity the PUMP beam was split into 2 equal-intensity beams which both passed through the sample cell, The difference in fluorescence intensity produced by the 2 beams was monitored while.the DUMP pulse, which passed only through one PUMP beam, was scanned in wavelength. This technique is relatively uncomplicated and employs the high sensitivity of fluorescence detection techniques, One disadvantage of this detection scheme arises from its sensitivity to processes other than the stimulation of emission from the excited electronic state which could also decrease the detected fluorescence. For example, the excited state produced by the PUMP pulse could absorb DUMP photons (instead of stimulating emission), thereby exciting the molecule to higher electronic states which rapidly predissociate or autoionize. This additional excitation process would effectively decrease the observed flwres::ence. For the case of I 2 and formaldehyde, the spectroscopic evidence clearly supports the almost exclusive occurrence of SEP . However, attempts to observe SEP processes involving the X~B2 state -76- of an:i:.line (~ee $ecttOP. 6)_ ~trongly :i:.nd:i:.cp,te th.e presence of up-:opumping by the ''DUMP'·' pul~e to a.uto·d.onizing or pre . . diss-ociative s·tates whi.ch competes effectively· with stimula.,. ting emission back to the ground electronic state. 5 There.,. fore, without a prior detailed knowledge of the spectroscopy of the molecule to be investi'gated and a careful investiga· tion of po~sible other fluorescence.,.decreasing processes, the fluorescence detection technique could give misleading results . A second method of monitoring the SEP process was em.,. ployed by Lawrence and Knight 2 ' 4 They demonstrated SEP as an effective method of populating vibrational levels of the ground electronic state in a medium-sized polyatomic molecule, p-DFB, 2 and used the SEP process as a preparative method to produce specific vibrational states whose lifetines were measured . 4 In these experiments the standard PUMP and DUMP pulses were followed by a third pulse, the PROBE pulse, which excited molecules prepared by SEP up to specific states whose fluorescence is detected , The PROBE pulse is effective in producing fluorescence only if the correct vibrational level has been prepared, This technique offers the advantage of specific detection of the level prepared by SEP, rather than assuming that a decrease in fluorescence corresponds to the preparation of a specific vibrational state. The possibility of partial absorption of the DUMP pulse and further excita 9 tion is not elininated, but by -m onitoring the population of the targeted final vibrational state, the extent of the -77- stimulated emission process is directly observed . The major disadvantage of this technique is the complexity of obtaining and overlapping 3 different laser pulses, both temporally and spatially, In addition, the spectroscopy of the molecule being studied must be known sufficiently to allow selection of an appropriate wavelength for the PROBE laser. The photoacoustic method for detection of SEP presented in the present paper does not specifically detect the population of the targeted vibrational state. Instead it monitors the excess energy remaining in the sample after both the PUMP and DUMP pulses. The PUMP pulse alone excites the molecule up to an excited electronic state which relaxes radiatively and nonradiatively. The nonradiative pathway, eventually leads,through collisions, to an increase in translational energy of the molecules which is detected as a pressure increase by a simple microphone . When the DUMP pulse stimulates emission out of the excited electronic state, the fraction of energy following the nonradiative pathway decreases, less energy appears as translational energy, and a smaller signal is detected from the microphone. If,on the other hand, the DUMP pulse is absorbed, more energy is deposited in the system and it is likely that the amount of energy follo·wing the nonradiatiye pathway will increase, increase in the microphone signal. This leads to an Therefore, by monitoring the increase or decrease in photoacoustic signal when the DUMP pulse inunediately follows the PUMP pulse, compared to -78- the PUMP pulse alone, it is possible to differentiate between excited state absorption and stimulated emission. The extent of the effect can be estimated if the relative quantum yields for nonradiative processes from the excited states are known . The ability to differentiate between absorption and stimulated emission provides a distinct advantage over the simple fluorescence detection method . The simplicity of the photoacoustic detection method compated to monitoring fluorescence due to a third PROBE pulse makes it an attractive alternative to detecting molecules in the targeted vibrational state directly . -7 9- 4. E2C.]erimental The two-color time-resolved photoacoustic measurement involved in monitoring SEP described here is a new application of photoacoustic spectroscopy. This same technique is direct- ly applicable to gas phase excited state absorption measurements with nanosecond or faster resolution, 5 (We are aware of only one other nanosecond timescale photoacoustic spectroscopy measurement, and this involved a solid phase sample.6} 4.1 Lasersand Optics The experimental arrangement is depicted in figure 1. It consisted of 2 counter-propagating collinear beams from frequency doubled dye lasers which passed through a photoacoustic cell containing p-DFB. The PUMP pulse was supplied by dye laser #1, a Molectron 7 DL200 dye laser pumped by a Molectron UV400 laser. The dye laser output was frequency doubled in a KDl' crystal, collimated with a 25 em focal length lens, and directed into the photoacoustic cell. Using Coumarin 485 dye in ethanol, typical UV pulse energies at the photoacoustic cell entrance were -15 X 10- 6 joules . The DUMP pulse was supplied by dye laser #2, a Molectron DL18 dye laser with an oscillator and single longitudinally pumped amplifier, pumped by a Molectron MY34 Nd:YAG laser. The output of dye laser #2 was also doubled in a KDP crystal and directed into the opposite end of the photoacoustic cell. A beam contracting telescope was used to match the beam -80- diameter of the doubled output of dye laser #2 to the -1 mm diameter of the dye laser #l's output~ Using R6G dye in ethanol, typical UV pulse energies before the photoacoustic cell were between 2 and 10 millijoules. UV outputs from either dye laser were monitored off beamsplitters with a 8 Laser l'recision pyroelectric joulemeter, model RkP-335, and readout unit, model Rk-3230. The analog output from the read- out unit was processed by a differential gated integrator system described later. Initial experiments indicated that the two dye lasers were interacting ~~th each other due to the collinear counterpropagating beam arrangement. This was eliminated by using Corning 7-54 UV pass filters to block the visible output after each doubling crystal, and Corning 0160 glass before each doubling crystal to keep the UV produced by one laser from entering the other. Wavelength scans of either dye laser were achieved using micro-computer controlled stepping motors to drive the gratings. The phasematching angle for the doubling crystal of dye laser 1 was adjusted manually during wavelength scans. The crystal for dye laser 2 was controlled by the micro-computer using a pre-determined 5th order polynomial in wavelength. Wavelengths were calibrated using a uranium/neon hollow cathode lamp and the optogalvanic effect. 9 UV output linewidths were ~ 1 em - 1 • 4.2 Timing and Wavelength Scanning The timing between laser pulses was controlled by a -81series of three delay generators. la~e;r Triggered by the Nd;YAG flashlamp sync signal~ the first delay generator pro~ duced a -160 ~sec delay which then triggered two other delay generators which triggered the Nd:YAG Q-switch and N laser thyratron, Long-term jitter between pulses, as monitored with a Hewlett Packard 5082-4220 PIN photodiode reverse bias, (~18 volt . <1 nsec), was less than ±2 nsec, with r1se short term (1.0 sec) jitter of ±1 nsec, Pulsewidths of the L doubled output of each dye laser were ~5 nsec. For experiments where the DUMP wavelength was scanned, the PUMP laser was operated at 20 Hz and the DUMP laser was fired only on alternating PUMP pulses. A differential gated integrator measured the difference in amplitude of the two photoacoustic signals from PUMP alone, and PUMP with DUMP. The integrator output along with pulse energy of the DUMP laser were digitized and stored on floppy disk. 4.3 Photoacoustic Cell Photoacoustic spectroscopy and the design of photo~ acoustic cells has been recently reviewed. 10 • 11 The photoacoustic cell used in this study was similar to that of West, et a1., 12 and modified slightly from our earlier studies. 13 The 18 ern long by ,9 ern diameter cell was constructed of stainless steel and aluminum. Quartz windows were held in place at each end using Viton o-rings . Since extreme sen- sitivity was not essential in working with the strong absorptions in p-DFB, the cell was designed to eliminate signals arising from scattered light striking the microphone or cell -82- walls, Since most light scattering occurs at the windows, this was minimized by a series of apertures both inside and outside the cell, The microphone, Knowles Electronics 14 BT1759, was located 6 em away from the PUMP entrance window to minimize absorption of the PUMP pulse before it reached the vicinity of ·the microphone. The microphone was positioned over a 3/16" diameter hole in the cell wall and held in place with epoxy resin. 4.4 Sample Preparation P-DFB, 98% purity from Aldrich, was subjected to several liquid N2 freeze-pump-thaw cycles and then vacuum distilled on a vacuum line operated at 5 X 10- 7 torr pressure, before filling the photoacoustic cell. Sample pressures ranged from 0.020 to 0.060 torr as measured by a Baratron (10 torr head), At the lower sample pressures helium was sometimes added to increase the photoacoustic signal through enhanced collisional deactivation. This had no effect on the % change in signal resulting from the SEP process. -83~1 Stimulated Emissjon Pum~!PK in R-Difluorobenzene We chose p-DFB to demonstrate the effectiveness of photo~ acoustic detection for monitoring SEP because of its extensively studied absorption and emission spectra and to allow comparison with previous SEP studies in the same molecule. 2 ' 4 5.1 Background Infrared and Raman, 15 and 2-photon 16 studies have deter- mined the ground state vibrational frequencies of p-DFB. The S1C 1 Bzu) excited state vibrational frequencies have been determined from absorption studies. 16 - 20 Single vibronic level fluorescence from 19 absorption bands has been studied. 21 Low pressure lifetimes and fluorescence quantum yields from single vibronic levels of S 1 have been measured. 22 - 24 Several notations have been used in labeling the ground state vibrations of p-DFB. We have used the standard Mulliken con- vention25 in this paper, which is also used in ref. 21. Transitions between vibronic levels in the ground electronic state and the first excited singlet ( 1 Bzu) are denoted by N~, where N is the number of the mode involved and i and j 1 are the number of quanta in mode N in the ground and excited electronic states,respectively. Figure 2 shows a photoacoustic spectrum of p-DFB produced by scanning the PUMP laser (no DUMP present). Assignments of the stronger transitions are given in table 1. In addition to the strong 0-0 band at 265,54 nm, the p-DFB spectrum predom~ inantly consists of progressions in Vs and v3, plus sequences 21 associated with members of these progressions. The 6~ . -84band is the stronge~t observed absorption involving quantum changes in modes other than vs and v3. 21 Single vibronic level fluorescence studies have identified the transition labeled k in figure 2 as involving excitation from the ground vibration state to an excited state Fermi resonance between states 5 1 and 6 2 , 21 with 5l contributing most of the transition intensity. 5.2 Results Figure 3 shows a photoacoustic SEP spectrum of p-DFB 2 produced by exciting the strong transition to the 5 1 and 6 resonant levels at 37659 cm- 1 with the PUMP laser and scanning the DUMP laser from 286 nm to 284 nm. The figure shows three scans taken with different delay times between the PUMP and DUMP pulses. When the DUMP pulse occurs 20 nsec before or 38 nsec after the PUMP pulse, no change in photoacoustic signal is observed. When the DUMP pulse follows the PUMP pulse by 4 nsec, the signal decreases by up to 10% at certain wavelengths, indicating that a SEP process is occurring at those wavelengths. The appearance of SEP only when DUMP immediately follows PUMP is consistent with the fluorescence lifetime of 10 nsec observed for the 5 1 level of S.1. 22 · "<:~-24 Photo- acoustic signal decreases as large as 15% have been observed. The per cent change depends critically on the spatial overlap of the two beams. No excited state absorption is observed for p-DFB for the DUMP wavelengths used. The SEP transitions observed in the 4 nsec spectrum of - 85- figure 3 are easily assigned by considering the p-DFB single vibronic level fluorescence spectra and ground state fundamentals given in ref. 21. In absorption and emission, the Frank Condon factors allow large changes in vibrational quantum numbers of only modes V3 and v 5 • 21 Quantum number changes for v6 are li'mited to ±1. Changes in quantum numbers in other modes produce trans it ions which are weaker by an order of magnitude. Table 2 lists the pos- sible ground electronic state vibrational levels which, according to Franck .. Condon arguments, are expected to be most intense in fluorescence from tne 5l or 6 2 levels populated by the PUMP laser. Each expected fluorescence transition is detected as a decrease in photoacoustic signal caused by SEP, although some transitions appear only as unresolved shoulders on the rotational envelopes of stronger transitions. The agreement between predicted and observed transition energies for the 3151~ and ±5 cm'" 1 53 levels is within expectations, considering the accuracy of the fluorescence data 21 and the difference in excitation bandwidth (28 cm- 1 for fluorescence vs. ~ 1 cm- 1 for SEP). 26 The relative SEP intensities agree well with the resolved fluorescence spectrum intensities after exciting the 51 level. 21 Figure 4 shows a time resolved photoacoustic spectrum of p-DFB obtained by varying the relative time between the PUMP and and DUMP pulses, while fixing their respective photon energies at 37659 cm- 1 and 35108 cm .. 1 • This excited the 51 and 6 2 levels of S1and stimulates emission down to the 315161 level -86- of the ground electronic state. The time dependence of the SEP effect detected as a decrease in photoacoustic signal correlate~ with the 10 nsec fluorescence lifetime22~24 of the 51 level of sl mentioned previously, if the laser pulse widths are taken into account. The wavelength and time dependence of the p-DFB SEP signal detected by the photoacoustic effect provide strong confirmation of the accuracy and utility of this new technique for monitoring SEP, -876. Photoacoustic Detection of Excited State Absorntion In Aniline In this section the ability to detect excited state ab- sorption, in contrast to stimulated emission, will be demon~ strated briefly by presenting data on aniline. Figure 5 shows a time-reso~ved photoacoustic spectrum of aniline obtained in a similar manner to that of the p-DFB spectrum of figure 4. In this case a 0.378 torr sample of aniline was used and the PUMP and DUMP photon energies were 34032 cm- 1 and 18797 em- 1 ' respectively. The PUMP pulse energy was 5. 4 ~J, and the DUMP pulse energy was 6.5 mJ . Under these conditions the PUMP excited the 0-0 transition 27-28 . from So to the S1 state, DUMP exc1ted no transitions from the ground state and cannot stimulate emission from · f rom S 1 up t o S l 3 2 and DUMP poss1. bl y may exc1. t e an1. 1 1ne ' . Sn, a state observed in vacuum UV a b sorpt1on stu a·1es. 3o ,.. 32 (DUMP is not the correct terminology for a pulse which promotes excited state absorption. Its use is retained here for comparison with SEP results.) In the spectrum of figure 5 a large increase in photo:acoustic signal of up to 70% is observed when the DUMP pulse follows PUMP by 0 to 20 nsec. When DUMP is delayed longer than 20 nsec, the signal decreases to a level that is still 20% higher than when DUMP precedes PUMP. The fluorescence lifetime of the vibrationless level o£ sl is approximately 8 33-35 and therefore the large increase from 0 to 20 nsec nsec , delay results from excitation of the singlet state by the -88- DUMP pulse. The longer apparent lifetime and increased sig- nal before 0 are a result o~ the width of the PUMP and DUMP pulses. The increase in signal for delays longer than 20 nsec most likely results from absorption from a long-lived triplet state produced by intersystem crossing. 36 The excited state absorption effects in aniline are presented in detail in ref. 5. 7. w Conclusions The results presented on p-DFB clearly demonstrate the sensitivity of the photoacoustic technique for monitoring SEP processes. The actual population transfer produced in these experiments was small due to the low pulse energy of the PUMP pulse. Increasing the PUMP pulse energy should only enhance the results reported here because of the in creased signal to noise ratio, Tha ability to detect excited state absorption, as demonstrated by the aniline study, and to distinguish that effect from SEP by simply monitoring the direction of change in the photoacoustic signal, makes this technique an attractive alternative to the fluorescence detection method. The simplicity of photoacoustic detection compared with the PROBE laserinduced-fluorescence technique should also be emphasized. The two-color time-resolved photoacoustic spectroscopy technique demonstat~d state spectroscopy. here is a powerful tool for excited The results on p-DFB and aniline dem- onstrates its ability to determine single vibronic level lifetimes, detect gas phase excited state absorption from both singlet and triplet states, and also monitor SEP processes. By using amplified picosecond laser, the temporal resolution could easily be extended to the picosecond regime. -90- TABLE I. Observed Vibronic Transitions in the p-DFB S 1 ( 1 B2 u ) Photoacoustic Spectrum Designation in Fig. 2 Vibronic Band Assignmenta Transition Energy (em- 1) a a 8i 365 94 b 30i 36726 c l?i 36740 d 3 0 22 36764 e 27 11 36797 f 3 0 11 368 02 g oo0 36840 h 610 372 48 i 5 10 30 22 37583 j 5 10 3 0 11 37621 k 510 (62) b . 0 37659 aFrom ref. 20 and 21. bFermi resonance mixes the 5 1 and 6 2 vibrational levels. The transition probability is predominantly derived from the 5 1 leve 1. 35055 35044 2604c 2615c 3163 Sz62 _, 35064 2595b 53 Using a PUMP photon energy of 37659 em 35094 256Sb 315161 d d 3507 3 35108 d Observed DUMP Transition Energies (em- 1 ) dTransition is present, but as a broad unresolved feature. Anharmonicity was not included. cDetermined from ground electronic state vibrational frequencies of ref. 21. bDetermined from 0-0 fluorescence data of ref. 21 a 35144 Predicted DUMP Transition Energies (em- 1 ) 251Sb Vibrational Energy (em - 1 ) p-DFB Vibrational Levels Populated by Stimulated Emission Pumping 3z Vibrational Band TABLE II. I I ...... 1.0 -92REFERENCES w 1. - c. Kittrell, E. Abramson, J. L. Kinsey, S. A. McDonald, D. E. Reisner 1 R. W, Field, and D. H. Katayama, J. Chern. - Phys. 75, 2056 (1981). 2. W. D. Lawrence and A. E. w-. Knight, J. Chern. Phys. z~, 5637 (1982). 3, D. E. Reisner, P. H. Vaccaro, C. Kittrell, R. W. Field, J . L. Kinsey, and H. -L . Dai, J. Chem. Phys. 4. 22, 573 (1982). W. D. Lawrence and A. E. W. Knight, J. Chern . Phys. 77, 570 (1982). 5, -- D. J. Moll, G. R. Parker, Jr. and A. Kuppermann,/'Two-Color Time-Resolved Photoacoustic and Multiphoton Ionization Spectroscopy of Aniline", to be published. 6. M. G. Rockley and J. Paul Devlin, Appl Phys. Lett . ~~' 24 (1977). 7. Molectron Corporation, 177 N. Wolfe Road, Sunnyvale, California 94086, USA. 8. Laser Precision Corp., 1231 Hart Street, Utica, New York 13502, USA. 9, R. A. Keller, R. Engleman, Jr. and B. A. Palmer, Appl. Opt i c s 12. , 83 6 ( 19 8 0 l • 10. "Optoacoustic Spectroscopy and Detection", Yoh-Han Pao, edit. (Academic Press, New York, New York, 1977). 11. A. Rosenwaig, "Photoacoustics and Photoacoustic Spec ~ troscopy", (John Wiley and Sons, New York 1980). 12, G. A. West, D. R. Seibert, and J. J, Barrett, J . Appl. Phys. g, 2823 (1980), -9313. D. J, Moll, J, W, Perry, A. Kuppermann, and A. H. Zewail, "High Overtone Photoacoustic Spectroscopy of Some MethylContaining Molecules. I. Rotational Analysis", to be published. 14. Knowles Electronics, Inc., 3100 N. Mannheim Rd., Franklin Park, Illinois 60131, USA. 15. 16. 17. 18, s. Green, Spectrochem. Acta A 26, 1503 (1970). M. J. Robey and E. w. Schlag, Chern. Phys. 19_, 9 (1978). c. D. Cooper, J. Chern. Phys. 3..?_, 503 (1954). s. N. Thakur and N. L Singh, Indian J. Pure Appl. Phys. - J. H, l ' 7 6 5 (19 6 9) • 19. Y. Udagawa, M. Ito, and S. Nagakura, J. Mol. Spectrosc. -36 ' 541 (19 7 0) • . 20. T. Cvitas and M. J. Hollas, Mol. Phys. 21. R. A, Coveleskie and C. S. Pararnenter, J. Mol. Spectrosc. ~' 1~, 793 (1970). 8 6 (19 81 ) . ~l' 22. C. Guttman and·S. A. Rice, J. Chern. Phys. 23. L. J. Volk and E. K. C. Lee, J. Chern. Phys. 24. D. Phillips, M. G. Rockley, and M. D. Swords, Chern. Phys. 661 (1974). ~' l§_, 301 (1979). 25. R. S. Mulliken, J. Chern. Phys. ~' 1997 (1955). 236 (1977). -9426, It appears that the calculated posit ions of the 3 °5t' 0 6 ~ ' 2 n sequence shown in fig. 6 of reference 22 are too high by 858 crn-J~ the v 5 fundamental frequency. o~o Also the single vibronic level fluorescence of the 5~ transi- tion is displaced 2595 crn~ 1 from 0~ fluorescence, corn~ pared to the calculated value of 2574 crn- 1 , indicating the possibility of perturbations of this level. These two clarifications bring the assignments of ref. 22 and this paper into agreement. 27. N, Ginsberg and F. A. Matsen, J, Chern. Phys. l~' 167 (1945), 28, J. C. D. Brand, D. R. Williams, and T. J. Cook, J. Molecular Spectroscopy~~' 359 (1966}. 29, M. Quack and M. Stuckberger, J. Molecular Spectroscopy ~' 30. 87 (1972)' K. Kimura, H. Tsubornura, and S. Nagakura, Bull. Chern. Soc. Japan ll' 1336 (1964). 2, 177 (1965). 31. K. Kimura and S. Nagokura, Molecular Phys. 32. K. Fuke and S. Nagakura, J. Molecular Spectrosc. ~' 139 (1977). 33. H. Von Weyssenhoff and F. Kraus, J. Chern. Phys. ~±' 2387 (1971). ~l' 34. W. R, Wave and A.M. Garcia, J. Chern. Phys. 187 (1974) , 35. R, Scheps, D.Florida, and S, A. Rice, 3. Chern. Phys. ~' 1730 (1974)' 36. K. C. Cadogan and A. C. Albracht, J. Phys. Chern. ~' 1868 (19 6 9 )_. -95- Figure Captions Figure 1. Experimental arrangement for photoacoustic detection of stimulated emission. The PUMP beam from dye laser #1 and the DUMP beam from dye laser #2 enter a photoacoustic cell collinearly from opposite directions. fined as follows: The various labels are de- A - aperture, BS - beamsplitter, Cl and C2 - KDP doubling crystals, Dl - pyroelectric energy probe, D2 - high speed photodiode, Fl and F4 - UV blocking filters, F2 and F3 - UV pass filters, HCL - Uranium/Neon Hollow Cathode Lamp, 11 and 12 beam contracting telescope, 13 - 25 em focal length lens, P - prism. Figure 2. Pulsed photoacoustic spectrum of a portion of the i 1Alg + ~ 1 B 2u transition in p-difluorobenzene. The sample consisted of 0.022 torr p-difluorobenzene and 3.32 torr helium. The photoacoustic signal has been corrected for the variation of incident laser pulse energy with wavelength. The vibronic bands labeled by the letters a through k are also listed Each letter designates the following 1 3 1 vibronic transitions: a 81, b - 303, c- 171, 1 0 1 1 d - 3o 22 , e - 271, f - 301, g - 00, h - 60, i 1 1 1 1 2 implies a The symbol sa. 50302, j 50301, k in Table I. - Ni transition with i quanta of mode N in the lower -96level and j quanta of mode N in the upper level. The modes are numbered using the convention of Ref. 25. Figure 3. Photoacoustic stimulated emission pumping spectrum of a 0.051 torr sample of p-difluorobenzene obtained by pumping the 5~ transition at 37659 cm-l and scanning the DUMP pulse wavelength. The upper and lower abscissae give the photon energy and wavelength of the DUMP pulse. The ordinates give the percent change in photoacoustic signal due to the stimulated emission process. The transition labels refer to transitions induced by the DUMP pulse. They are described in the caption for Fig. 2. (a) The DUMP pulse precedes the PUMP pulse by 20 nsec. (b) The DUMP pulse occurs 4 nsec after the PUMP pul~e. (c) The DUMP pulse occurs 38 nsec after the PUMP pulse. In each spectrum the signal shown is the difference in photoacoustic signal produced by PUMP alone and PUMP plus DUMP. The difference signal has been corrected for variations in the DUMP laser pulse energy during the scan. Figure 4 . Time-resolved stimulated emission pumping photoacoustic spectrum of a 0.051 torr sample of pdifluorobenzene. The PUMP pulse excites the s 01 transition at 37659 cm-l The DUMP pulse stimulates emission out of the s 1 -6 2 resonant levels down to - 97'- the 3 1 51 6 1 level . -The spectrum was generated by scanning the delay between the PUMP and DUMP pulses. The abscissa gives the time delay between the two pulses, ~t = tDUMP - tPUMP' The ordinate gives the percent change in photoacoustic signal due to the stimulated emission process. The photo- acoustic signal has not been corrected for variations in the PUMP or DUMP pulse energies. Figure 5 . Time-resolved pulsed photoacoustic spectrum of a 0.378 torr sample of aniline. The PUMP photon to s 1 0-0 transition at 34032 cm- 1 . 0 The DUMP photon energy was 18797 cm- 1 . The spectrum excited the s was generated by scanning the time delay between the PUMP and DUMP pulses. The abscissa gives the time delay between the two pulses, ~t = tDUMP - tPUMP' The photpacoustic signal has not been corrected for variations in the PUMP or DUMP pulse energies. -98- D1 DYE LASER #2 Nd:YAG LASER C1 L1 L2 --A HCL MPI OR PAS CELL D1 D2 C2 F4 N2 LASER t----~ DYE LASER #I Figure 1 'Tl - 0 ..c. 0.. 0 - 0 0 u 0 :::J CJ) - u ·- Ol N c t-j -0 "-... <( '- ·..a '- (1) ~ 1-'· QQ ~ '- 0 r ~ I' 274 aI 272 l II bed ef ~ II I II 270 268 Wavelength/ nm ~ 1\ Jl i~ 266 j I 264 p-DFB I I \0 \0 -100Wavenumber /cm- 1 35100 35000 35200 (a) ;:.!? 0 (b) '~ 0" iJ'J · u -B\ :::l 0 u 0 0 0 ..J::. ~ .~ <1J 0" 8 ..J::. u (c) 5 286.0 285.5 285.0 Wavelength /nm Figure 3 284.5 284.0 -101- ~------------~~------------------------,~ CD LJ... a I 0.. 0 rt) 0 N 0 u Cl> 0 Vl c - ....... <J 0 I 0 N I 0 rt) I 0 ~ I 0 0 I{') I 0 I %I IOU~S :>qsno::>DOl04d U! a5UD4J Figure 4 I{') I -102- ----------~------------------~8 w z _J z <t 0 LO I 0 ~~--~~~--~~~~~--~~~o Q 8 S+!un AJDJ+!qJ'i/lou5!s :>!+sno:>oo+04d Figure 5 OT -103- Paper III: Two-Color Time-Resolved Photoacoustic and Multiphoton Ionization Spectroscopy of Aniline. -1041wo-Color Time-Resolved Photoaco~stjc and MultiQhoton Ionization SQes:.t.r<?SCOJ2X of Anj.lil}~a) D. J. Mollbl, G. R. Parker, Jr. and A· Kuppermann Arthur Amos Noyes Laboratory of Chemical Physicsc) California Institute oi Technology, Pasadena, CA 91125 USA (Received The multiphoton ionization and photoacoustic signals from aniline have been detected as a function of time delay between two laser pulses of different wavelength. pulse excited the S 1 0-0 transition. The first The second pulse excited either S 1 or a triplet state produced by intersystem crossing, depending on the delay time between pulses. Both ionization and dissociation processes are observed. By varying the con- ditions of excitation it appears that a given amount of energy can be selectively channeled almost exclusively into either ionization or dissociation pathways. The results are explained using a simple Franck-Condon factor model. a)Research supported in part by the U. S. Department of Energy, (Contract No. DE-AS03-765F00767, Project Agreement No. DE-AT03-76ER720004). b)Work performed in partial fulfillment of the requirements for the Ph. D. degree in Chemistry at the California Institute of Technology. c)Contribution No. J. -105- INTRODUCTION In typical multiphoton ionization (MPI) experiments, an intense UV or visible laser pulse is focused into a gaseous sample, producing an ionization current which is measured as a function of wavelength. When integral multiples of the photon energy are resonant with states in the sample an increase in ionization current occurs. Recent MPI experiments have detected the masses of fragments produced and/or the kinetic energy of released electrons in molecules such as h d 8 No , 1,2 I 2 , 1,3 H2 S , 4 NH 3 , 5 b enzene, 1,3, 6,7 aceta ld eye, an1"1"1ne, 9 azu 1 ene, 10 nap h t h a 1 ene, 10 an d organometa 11"1cs. 11 Three mechanisms have been proposed to explain the observed fragmentation: (1) multiple absorption of photons in neutral valence, Rydberg, and autoionizing states to produce super- excited states which eventually autoionize and dissociate into the observed fragments, (2) initial preparation of the parent ion by multiphoton absorption with subsequent absorption of more photons to dissociative or predissociative ionic states, and (3) dissociation from the neutral manifold before reaching the ionization potential (IP), followed by ionization or further dissociation of the fragments. There is strong evidence, particularly from MPI photoelectron studies, that indicates that most molecules follow mechanism (2) where ionization to the parent ion occurs as soon as sufficient energy to reach the IP of the molecule has been absorbed. Mechanism (1) has only been reported in a few scattered cases, as a minor 2 process in ·MPI through the A 2 ~+ (v•3) state of N0, and in - 106- azulene. 10 Mechanism (3) has beeJ'l observed in weakly bound . organometa 11 1c systems, 11 This paper presents two-color time-resolved photoacoustic and MPI results on aniline which strongly support the presence of mechanism (3) in that molecule for excitation from a state prepared by intersystem crossing , In this specific case, the molecule is excited above the adiabatic IP, but dissociates almost exclusively into neutral fragmentsrather than ionizing. By varying the conditions of excitation in aniline, a given amount of energy can be channeled almost exclusively into either ionization or dissociation , Aniline was initially selected for these experiments because of our interest in photoacoustic detection of stimulated emission pumping . 12 After initial excitation to a vibronic level in S 1 we observed only additional excitation to higher excited states, when a second laser pulse was introduced instead of stimulating emission back down to a vibrational level of the ground electronic state. An investigation of the excited state absorption phenomena is reported in this paper . -1072. Back_g_round The. electronically excited states of aniline have been . d b y a b sorpt1on, . 13-20 em1ss1on, . . 21-30 and extensively stu d 1e multiphoton ( 1 . 31-35 techn1ques. The lowest excited singlet B2 ) with a 0-0 transition at 34032 cm - 1 exhibits a sharp single vibronic level absorption spectrum with a broad underlying continuum . Fluorescence lifetime and quantum yield measurements of single vibronic levels indicate a fluorescence lifetime ranging from 5 . 8 to 9.8 nsec, and a quantum yield for fluorescence ranging from 0.1 to 0.3. 24 The 0- 0 level of S 1 , which is used in this study, has a fluorescence lifetime and quantum yield of 8.57 nsec and 0.30, respectively. 24 T 1 phosphorescence with a lifetime of 4.2 seconds has been observed from the low-lying 3 A1 state in aniline in solid solution at 77° K after exciting S 1 • 28 The ratio of phosphorescence to fluorescence quantum yields was 3.1, and total luminescence quantum yields were estimated at close to one. 28 An additional low-lying T state ( 3 B ) was pos2 2 tulated to be isoenergetic with S 1 to explain early fluor. d ata, 21 b ut no d"1rect ev1"d ence f or th e escence 11.£ et1me location of that state has appeared. The energy of T 1 was determined from phosphorescence which places that state about 7200 cm- 1 below sj . 27 Extrememly efficient 2-photon ionization has been observed for aniline using ~as an intermediate state, made possible by the low IP (62,100 cm- 1 36 ) and the fully allowed · · b etween S o an d 5 1 • 31 , 32 At 1·ntennature o f the transrtrons -108sities below 10 megawatts/cm 2 the parent ion accounted for When increased to 20 megawatts/cm 2 the parent ion still accounted for 85% of the ion signa1. 32 100% of the ion signal. -109- }· Ex£erimental The two-color time resolved photoacoustic and MPI ex- periments reported in this paper involved subjecting a gas phase sample of aniline to two counter-propagating collinear laser pulses with different wavelengths and variable delay between them, adjustable with nanosecond precision. ~t The re- sulting total ion current was measured as a function of 6t at different wavelengths using a simple parallel plate MPI cell. The energy deposited into the sample which was quickly transformed into translational energy of the gas molecules was detected as a pressure wave by a microphone in a photoacoustic cell. The amplitude of this pressure wave was also measured as a function of ~t at different wavelengths. Specific de- tails of the experiments are discussed in the following sections. 3.1 Optics and Laser Systems The experimental arrangement is shown in fig. 1. It consisted of 2 counterpropagating collinear beams from frequency doubled dye lasers which passed through either a photoacoustic cell or an MPI cell containing a gas phase sample of aniline. the Sl The PUMP pulse used to initially populate state was supplied by dye laser #1 (DLl), a Molectron 37 model DL200 dye laser pumped by a Molectron UV400 N2 laser. The output of DLl was frequency doubled in a KDP crystal, collimated with a 25 em focal length quartz lens, and directed into the sample cell . Using rhodamine 6G dye in ethanol, -110typical UV pulse energies ranged from 0.5 to 10 ~J at the sample cell entrance . The TRANSFER pulse which transferred population from S 1 to a higher excited state was ~upplied by dye laser #2 (DL2), a Molectron model DL18 dye laser with an oscillator and single longitudinally pumped amplifier, pumped by a Molectron MY34 Nd:YAG laser . In some experiments the transfer pulse was the 2nd or 4th harmonic of the Nd : YAG laser . The TRANSFER pulse was directed into the opposite end of the sample cell after passing through a beam contracting telescope which matched the beam diameter of the doubled DL2 output to the 1 mm diameter beam of DLl. PUMP and TRANSFER pulse energies were monitored off . ·~ Laser PrecJ.s1on . . 38 pyroe 1 ectr1c . JOU . 1 emeter, b ~amsp 1 1tters w1:u1a model RkP-335, and Rk-3230 readout unit . The analog output from the readout unit was processed by a differential gated integrator system described later. Linewidths of all laser outputs were <1 cm- 1 • Initial experiments indicated that the 2 lasers were interacting with each other due to the collinear counterpropagating beam arrangement. This was eliminated by using Corning 7-54 UV pass filters to block the visible output after each doubling crystal, and Corning 0160 glass before each doubling crystal to keep the UV produced by one laser from entering the other laser. 3. 2 Timing Between Laser Pulses The accuracy of the time delay ~t= ttransfer - tpump -Ill- between the PUMP and TRANSFER pulses and the ability to continuously scan 6t were critical to the performance of these experiments. Three delay generators were used to control the timing between the 2 pulses, The first was triggered by the Nd:YAG flashlamp synchronization output and generated most of the 160 ~sec delay before the Q-switch normally fires. After this initial delay, two fast-risetime delay generators (Tr < 10 nsec)were used to independently trigger the Nd:YAG Q-switch and the Nzlaser thyratron. wng-term jitter in 6t, as monitored with a Hewlett Packard 5082-4220 PIN photodiode (-18 volt reverse bias, Tr < 1 nsec), was less than ±2 nsec, with short term (10 seconds) jitter of ±1 nsec. Pulsewidths of the doubled output of each dye laser were ~5 nsec, Nd:YAG 2nd and 4th harmonic pulsewidths were -10 nsec, The delay 6t between pulses was continuously scanned using a 1 revolution/hour clock-drive motor to turn the vernier delay knob of one of the delay generators. During the scan, absolute values of 6t were monitored with the photodiode and a 10 nsec risetime oscilloscope. The final data were replotted using a time axis generated by a polynomial fit of the 6t data points to correct for a small nonlinearity in the 6t scan system, The accuracy of thetime values shown in the figures is estimated at ±2 nsec, 3.3 Photoacoustic Cell Photoacoustic spectroscopy has been used in a variety of applications, Excellent reviews of the technique can be -112found in ref, 39 and 40. The photoacousticcell used in these experiments was similar to cells used previously. 41 , 42 The 18 ern long by .9 ern diameter cell was constructed of stainless steel and aluminum. Quartz windows were held in place at each end using Vitron o-rings. Since extreme sensitivity was not essential in working with the strong absorptions in aniline, the cell was designed to eliminate signals arising from scattering light striking the microphone or cell walls. Since most light scattering occurs at the windows, this was minimized by a series of apertures both inside the cell and out, The microphone, Knowles Electronics model BT 1759, 43 was located 6 em away from the PUMP entrance window to minimize absorption of the PUMP pulse before it reached the vicinity of the microphone. The microphone was positioned over a 3/16" uiarneter hole in the cell wall and held in place with epoxy resin. 3.4 MPI Cell The MPI cell was a simple biased parallel plate-type 44 cell which has been described elsewhere. Briefly, it consists of a stainless steel cell containing two parallel plates 0.75 in. square, separated by 0.75 in. and quartz windows. The plates are biased with 22.5 volts. The charge pulse is detected by a Canberra 2001A charge-sensitive preamplifier with a sensitivity of lOV/picocoulornb. The slow decay time of the preamplifier (SO ~sec) allows total integration of the charge produced by 2 laser pulses for I~< 1 ~sec. -1133 , 5 Signal Processing Signals from either the photoacoustic cell or the MPI cell were limited to a frequency range of typically 100 to 8 x 10'+ Hz by an electronic filter and then amplified, The peak of the first pressure wave of the photoacoustic signal or the peak of the MPI signal from the charge sensitive preamplifier were integrated by a differential gated integrator system with an integration window of 15 ~sec. Signals from the pyro-electric joulemeter were processed in a similar manner by a second integrator system, A microcomputer digitized and stored the signals from both integrators at periodic time intervals approximately equal to the time constant of the gated integrators, as ~t was scanned, 3,6 Sample Preparation Aniline was obtained from Mallinckrodt and subjected to a minimum of 4 liquid N2 freeze-pump-thaw cycles and a vacuum distillation on a vacuum line operated at 5 xl0- 7 torr before use. Samples were stored in darkness. When filling the MPI or photoacoustic cells, the final pressure was allowed to stabilize for several minutes before closing the cell valve to minimize wall absorption effects, 4. -114- Results The photoacoustic spectrum of a 0.37 torr sample of aniline in the region of the S1 0-0 transition is shown in figure 2. Several of the prominent bands, including the 0-0 transition, are labeled according to the assignment of reference 14. This spectrum was obtained by scanning the wavelength of DLl. The results presented in the following sections were obtained by pumping the 0~0 transition at 34032 cm- 1 with the PUMP pulse and measuring- the photoacoustic and MPI signals as a function of ~t for TRANSFER pulse wavelengths of 532 nm, N305 nm, and 266 nm, The 0-0 transition was chosen for PUMP to avoid the effects of vibrational energy redistribution within S1. Preliminary studies where higher vibrational levels of S1 were pumped gave similar results to those described below. 4,1 532 nm TRANSFER Pulse Fig. 3a shows the MPI time-resolved spectrum of aniline obtained by exciting the S1 0-0 transition at 34 1 0.32 cm'" 1 with the PUMP pulse, using the Nd;YAG 2nd harmonic at 532 nmas the TRANSFER pulse 1 and varying ~t. For l~tl ~ 20 nsec the MPI signal is due almost entirely to a 2-photon ionization with a one-photon resonance in S1 , induced by the PUMP pulse, The efficiency of this process required reducing the PUMP pulse energy to 0,5 ~J/pulse to avoid saturating the charge sensitive preamplifier, which occurs at ion yields greater than 10 7 ... 115.,. ions/pulse. The 2 mJ transfer pulse produced no detect- able ions at these sensitivities. For I fit I < 5 nsec, i.e. both pulses overlapped in t irne, a sharp decrease in ion yield greater than 25% is observed. This effect has been previously observed under beam conditions and attributed to competition between ionization from S 1 induced by a second PUMP photon, or excitation from S 1 to a rapidly predissociating state induced by the 532 nrn TRANSFER pulse, as shown in figure 4. 9 The predissociation process is apparently fast enough that the molecule dissociates before another TRANSFER or PUMP photon can be absorbed to ionize the molecule. This mechanism is supported by VUV absorption studies 16 ,l?,lg which show strong absorptions near 52,800 cm- 1 , the presence of a photodecomposition film on the windows after extended data collection, and the photoacoustic measurements presented next. The N-H bond energy is only 28,000 cm- 1 , . . t h e 1 owest energy d"lssoc1at1on . . rna k 1ng 1t pat h way. 45,46 Figure 3b shows the corresponding photoacoustic timeresolved spectrum. An increase of almost 70% in photoacoustic signal is observed near At=O. In addition, the signal for At> 20 nsec (TRANSFER pulse after PUMP pulse) is about 20% larger than for At< -20 nsec (TRANSFER pulse before PUMP pulse). The pulsed photoacoustic technique is sensitive to energy deposited in the sample, but only to energy which is rapidly transformed into translation energy on the ~sec timescale. Since the photoacoustic signal increased substantially for values of At where the ionization signal decreased, the photo- ""116 .. acoustic detect ion technique appears to be much more sensitive to the dissociation process than to the ionization process. It is possible that the dissociation process channels a detectable amount of energy directly into translational energy of the fragments. Pressure waves from the vacuum UV photodissociation of 02 have been predicted theoretically 47 and observed experimentally. 48 Non-dissociative excited states such as S1 require collisional deactivation before the energy is released, a process which may be less efficient on the ~sec timescale for pressures less than 0.5 torr. The ion- ization process may also be inefficient depending on re~ combination and electron attachment rates. The time depen- dence of the actual photoacoustic signal also supports the channeling of excitation energy into kinetic energy of the fragments. For values of ~t in fig. 3b where an increase in photoacoustic signal is seen, the pressure wave appears at the microphone about 2 ~sec earlier than for excitation of S1 alone or for At< .. zo nsec. (In aniline at 0.37 torr we normally detect the pressure wave about 65 ~sec after the laser pulse. This delay results from both collisional de- activation times and the transit time of the pressure wave from the laser beam to the microphone.}. The 70% increase in photoacoustic signal for At~ 0 also supports the efficient release of kinetic energy through dissociation. For equal efficiencies of translational energy release the maximum increase in signal possible (every molecule excited by PUMP is also excited by TRANSFER) is 55%, bas-ed on photon energies. -117To produce a 70% increase, a mechanism like dissociation must be occurring which releases translation energy more efficiently than the collision·al deactivation of sl. In summary, the experimental evidence for PUMP and TRANSFER (532 nml producing dissociation is: Cl) an increase in photoacoustic signal occurs when a decrease in MPI signal is observed, (2) the pressure wave is detected about 2 ~sec earler, and (3) the signal increase caused by the TRANSFER pulse is larger than predicted by photon energy considerations alone. From fluorescence and phosphorescence quantum yields and lifetimes it is known that the initially excited 0-0 level of S 1 disappears with a lifetime of 8.5 nsec due to intersystem crossing to a nearly triplet state, which is probably the 3 A state, and fluorescence. 24 , 28 The increase in photo1 acoustic signal near ~t=O in fig, 3b is therefore due to excitation to a predissociative state from the 0-0 level of S 1 prepared by the PUMP pulse. For ~t > 20 nsec, the increase must be caused by triplet-triplet absorption frcm the state prepared by intersystem crossing from S 1 • Triplet-triplet absorption in aniline has been observed from 320 to 600 nm in rigid solution at 77° K. 49 OUr gas phase photoacoustic results indicate an absorption at 532 nm from a vibrationally excited triplet state to a predissociative triplet state. Fig. 4 is a diagram showing the processes occurring with the 532 nm TRANSFER pulse. llS .. The differences in widths of the features at At=O in ~ figure 3a and 3b result from two different processes. In figure 3a, the MPI spectrum, there is competition between the ionization process occurring during the PUMP pulse. In figure 3b, however, the large increase in photoacoustic signal is produced from the sl population which has a longer lifetine than the PUMP pulsewidth. 4,2 30S nm TRANSFER Pulse Figure Sa shows the MPI time- resolved spectrum of aniline obtained under conditions similar to the spectrum of figure 3a, except that now the TRANSFER pulse is near 30S nm. At this wavelength the TRANSFER pulse's photon energy falls just below the S 1 hot bands, but in combination with the 34032 cm- 1 PUMP pulse, the molecule can be excited to about 4700 cm- 1 above the adiabatic IP of 62100 cm- 1 . 36 In figure Sa the MPI signal shows an 8-fold increase in yield near ~t=O which decays exponentially with a time con- stant of 8. 9 ± 0. 2 nsec back to the ~t < 0 signal level. This 8-fold increase is a strong contrast to the decrease observed using the S32 nm TRANSFER pulse. (The periodic oscillations in the decay curve are not reproducible and result from jitter in the PUMP pulse timing.) The figure Sa spectrum is insensitive to pressure over a range of 0.004 to 0.37 torr, and the decay time constant should be close to the 8.S nsec fluorescence decay constant 24 of S1 if laser pulsewidths are considered. This implies that after intersystem crossing .-119 .. from the 5 1 0-0 level, the TRANSFER pulse is no longer capable of e£ticiently ionizing the molecule . From energy considerations alone there is no reason to expect the large decrease in ion yield for 6t > 10 nsec, since complete collisional deactivation of the vibrationally excited triplet produced by intersystem crossing is unlikely at 0 . 004 torr for timescales less than SO nanoseconds, and there is no inherent reason why ionization from triplet states should be difficult. 44 Consideration of vibrational effects offers an explanation of the inability to ionize the state produced by intersystem crossing. The initial excitation of 5 1 produces a state whose energy is almost 100% electronic energy. The vibrational wavefunction for this state is a zero point energy wavefunction. During intersystem crossing to the 3 A1 state, the total energy remains the same, but 7200 cm- 1 is converted from electronic to vibrational energy. The new vibrational wavefunction corresponds to a highly excited vibrational level with an energy of 7200 cm- 1 • The probability of direct ionization invoLves the Franck~Condon factor between the ion and the level of the neutral molecule from which ionization occurred . Therefore for small changes in geometry between 5 1 , T1 , and the ion, the vertical IP from 5 1 is expected to be about 7200cm-l lower than thevertical ~p from the state produced by intersystem crossing. Larger changes in geometry may decrease the 7200 cm- 1 di££erence, but it is still likely that the 305 nm TRANSFER pulse can only ionize the 5 1 state, -12 0- and not the vibrationally excited triplet state, by a direct ionization process. Even if Franck~Condon factors make the direct ionization process highly unlikely, there is still the possibility of absorption to a triplet quasidiscrete level above the adiabatic IP that rapidly undergoes an autoionization process. - Photoelectron energy analysis following MPI through the B state in ammonia provides an example of vibrational autoionization occurring from quasidiscrete levels. 5 A triplet quasidiscrete level must exist in aniline near 66800 cm~ 1 if the absorption from T 1 is to occur. Figure Sb shows the time-resolved photoacoustic spectrum corresponding to the conditions of figure Sa. are immediately obvious. Two features First, the sharp increase in MPI signal observed in figure Sa does not appear. This agrees with the result of the prev·ious section that the photoacoustic technique has a low sensitivity to ionization. It also implies that substantial amounts of dissociation are urllikely, in agreement with previous aniline MPI fragmentation studies. 31 , 32 Second, an inverse exponential increase occur-s, starting near ~t=O, and rising to a 40% increase in signal on a timescale similar to the decay observed in the figure Sa MPI spectrum. Figure 6 shows a spectrum similar to figure Sb which covers a larger range of ~t. The photoacoustic signal increase for ~t > 0 is long-lived, lasting at least 10-lS llsec, The observed decay may be an artifact of our photoacoustic detection technique caused because the pressure waves created by the PUMP and TRANSFER pulses no longer arrive at the ·d21~ microphone within the integration gate, or because of diffusion of the initially excited molecules out of the laser beam region. The decay rate did not change when an electric field of 80 volts/em was applied, indicating that the increase in photoacoustic signal is not caused by absorption of the TRANSFER pulse by ions resulting from the PUMP pulse. The photoacoustic results imply that the triplet state produced by intersystem crossing is long-lived (>10 ~sec) in the gas phase, and that it does pulse. absorb~ the 305 nm TRANSFER This absorption must populate a quasidiscrete level above the adiabatic IP which, according to the MPI results, does not ionize. Autoionizing molecular states are expected to have lifetimes 50 shorter than 10- 13 sec. The only process that could compete effectively with the formation of ions on that timescale would be a fast dissociation process. H2 photodissociation quantum yields of 10% have been measured both for single photon absorption above the IP, 51 and MPI processes above the IP. 52 The photoacoustic pressure wave reaches the microphone about 2 ~sec earlier for 6t > 10 nsec, as expected for a dissociation process, as discussed in section 4.1. Further support for the dissociation mechanism comes from the photoproduct analysis discussed in section 5. Figure 7 depicts the processes involved in the 305 nm TRANSFER pulse experiments. It is interesting that a given amount of energy can be deposited into aniline and selectively channeled either into ionization or dissociation channels almost exclusively~ depending on the way in which the ' 12 2 " energy is put into the molecule, Pumping the molecule up through the singlet manifold channels the energy into ionization, Introducing a delay of ~10 nsec in the middle of the excitation process allows an intersystem crossing process to select a new electronic manifold of states and converts some electronic energy into vibrational energy . The addition- al excitation new is direeted into a dissociative channel. Attempts were made to detect stimulated emission in aniline using the photoacoustic detection technique as was demonstrated in ref. 12. By using the PUMP laser to excite the 0-0 level of S1 and scanning the TRANSFER laser from 305,5 nm to 300.5 nm with 6t=2-4 nsec, we hoped to stimulate emission using the fluorescence lines observed near 302.5 nm.26 No decreases in photoacoustic signal were observed. Instead, we detected weak hot band absorption to S1 and a small wavelength independent increase in photoacoustic signal caused by the excited state absorption from the small triplet population produced at the values of ~t we used. 4.3 266 nm TRANSFER Pulse Figure 8 shows a time-resolved MPI system obtained by again pumping the 0-0 transition of SJ, but using a TRANSFER pulse at 266 nm, the 4th harmonic of the Nd:YAG laser. Excitation by both the PUMP and TRANSFER pulses brings aniline about 9500 cm- 1 above its adiabatic IP, enough excess energy to ionize either S 1 or T 1 by a direct ionization process, As expected, for all values of ltt1>-0 an increase in MPI "1' 123..- signal is observed. For 6t near 0 a 100% increase in MPI signal is observed, corresponding to excitation from S 1 • For 6t > 25 nsec, a 15% increase in MPI signal occurs, implying that the triplet is finally being ionized, The sig· nal to noise ratio in the spectrum of figure 8 is low, but the increase in signal for 6t > 25 nsec was reproducible in the several scans attempted, One could ask why the 266 nm pulse does not excite molecules which can be ionized by the PUMP pulse for ~t < 0. Photoacoustic signals produced by the 266 nm pulse alone arrive at the microphone 2-3 ~sec earlier than for excitation of the 5 1 0-0 transition. 266 nm excites aniline to a state which has an energy about 9,600 cm-1 above the N-H bond energy, and the absorption spectrum has become broad and diffuse at that energy. 24 Therefore we tentatively feel that the state excited by 266 nm alone may predissociate with a reasonable quantum yield, decreasing the ion yield from subsequent ionization by the PUMP pulse for 6t < 0. The photoacoustic time-resolved spectrum using the 266 nm transfer pulse is almost flat for -100 < 6t < 100 nsec for conditions similar to those in figure 3b or figure 5b. A slight increase of -1% may occur for 6t > 0, but this effect is almost obscured by noise, The 266 nm pulse alone, as mentioned earlier, does produce a pressure wave which arrives at the microphone 2-3 ~sec be£ore the pressure wave due to excitation of the ~ 0-0 transition, tion. This may imply dissocia~ From energy considerations and the MPI spectrum of -124- figure 8 we expect molecules excited by both the 266 nm and 305 nm pulses to predominantly ionize, Since the photo- acoustic technique is not highly sensitive to ionization in aniline, the increase in signal from excited state absorption is expected to be small. ?· .-12 5 .. ~hoto~roduct Detection The two alternative pathways described in section 4t2 lead predominantly to ions or neutral fragments, depending on the excitation scheme employed. spectrometric We attempted a mass product analysis after irradiating under case (~t= (A) conditions, which should produce predominantly ions 0-5 nsec, TRANSFER pulse A= 305 nm),or under case (B) conditions which should produce neutral fragments TRANSFER pulse A= 305 nml. (~t=lOO nsec, In both cases, the PUMP pulse was tuned to the S1 0-0 transition. Two new matched quartz ab- sorption cells ( 1 inch diameter x 4 inches long) with teflon stopcocks were filled simultaneously with typically 0.37 torr aniline, and 0.10 torr He for a reference gas, Irradiation times were 45 minutes at 10 Hz repetition rate with pulse energies of about 3 ~J and 500 ~J for PUMP and TRANSFER pulses, respectively. After irradiation, the He/Hz ratio was mea- sured after liquid Nz trapping of other photoproducts, and also the entire mass spectrum of the samples was recorded. Preliminary results indicate that more Hz is produced for case (B) conditions than for case ~1\1. Case (B) conditions also produce a small amount of product with a m/e peak at 114, which is not produced under case (A) conditions, H2 could be produced by breaking a N-H or c.. H bond, followed by the H atom extracting another H atom during a reactive collision. The mass 114 peak is ·more difficult to explain. Several molecules with the formula C 6 H 1 ~N 2 are known to give strong mass 114 peaks, 53 , 54 but producing these molecules -126~rom aniline requires breaking the aromatic ring and addition of more H atoms. The results presented here are only prelim - inary, and we are investigating other more direct ways of monitoring the fragmentation process in aniline. 6. Conclusions .-127 .. The two-color time-resolved photoacoustic and MPI results presented in this paper give indirect but strong evidence for both ionization and dissociation occurring in aniline for energies slightly above the adiabatic IP. By simply varying the delay time ~t between the PUMP and TRANSFER pulses the energy can be selectively channeled almost exclusively into either ionization or dissociation. li~ved This selectivity is be- to occur because conversion of a large amount of elec- tronic energy into vibrational energy takes place during intersystem crossing, and thereby alters the Franck-Condon factors for direct ionization and increases the energy of the vertical IP, The intersystem crossing also transfers the electronic energy into a different manifold of electronic states. The ability to channel a given amount of energy selectively into either ionization or dissociation channels is an exciting possibility for the . field of laser selective chemistry. The decay of ionization signal during intersystem crossing does not seem to be an artifact of aniline alone. Fig. \ 9 shows preliminary results on benzene obtained by pumping the X1 A1 g ~ A1 B u v\ -0 transition in benzene, 55 and using a 266 nm 2 8 TRANSFER pulse, The ionization signal decays exponen- tially with a time constant of 82 ± 2 nsec, in close agreement 56 to the observed fluorescence lifetime of 79 nsec. Previous two-color MPI experiments on azulene and benzophenone, however, did not detect exponential decays in ionization signal,10 Since the franck Condon factor explanation of the -128- observed MPI signal dec~ys holds for internal conversion as well as intersystem crossing, the two~color time-resolved MPI technique may be a powe.r ful tool for investigating ultrafast internal conversion processes using picosecond lasers, Additional experiments are needed to completely understand the excited state dynamics in aniline near its IP . 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Benson and H. E. O'Neal, Kinetic Data on Gas Phase Unimolecular Reactions, National Bureau of Standards Washington, DC, NSEDS-NBS 21, 1970. 46} J. A. Kerr, Chern. Rev.§§, 465 (1966). 47) W. Zuzak and B. Ahlborn, Physica 41.., 193 (1969). 48) R. C. Cross and R. Ardila, Can. J. Phys. ~' 2640(1970). 49) K. C. Cadogan and A. C. Albrecht, J. Phys. Chern. ZJ, 1868 (1969)_. SO) J. W. Rabalais, Principles of Ultraviolet Photoclectron Spectroscopy (Wiley, New York 1977), p. 78. 51) J. E. Mental! and E. P. Gentieu, J. Chern. Phys. ~' 5641 (197 0). 52) S, T. Pratt, P.·M. Dehmer, and J, L. Dehmer, :'Resonant Multiphoton Ionization of H2 via the B 1 ~+u' v=7 ' .J=2 . and 4 levels with Photoelectron Energy Analysis'~ submitted to J. Chan. Phys. 53) A. Cornu and R. Massot, Compilation of Mass Spectral Data, Vol. 2, 2nd edit. (Heyden and Sons, New York, 1975}, 54) S. R. Heller and G. W. A. Milne, EPA/NIH Mass Spectral Data Base, Vol. 1, NSRDS-NBS63 (19781, 551 G, Herzberg, Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules, (Van Nostrand Reinhold Co., New York, 1966), p. 178. -133- 56) K. G, Spears and S. A. Rice, J, Chern. Phys. ~ 5561 (1971). -134- Figure Captions Figure 1. Experimental arrangement for two-order time-resolved photoacoustic and multiphoton ionization experiments. The PUMP beam from dye laser #1 and the TRANSFER beam from dye laser #2 enter a photoacoustic spectroscopy (PAS) or multiphoton ionization (MPI) cell collinearly from opposite directions. labels are defined as follows: The various A - aperture, BS - beamsplitter, Cl and C2 - KDP doubling crystals, Dl - pyroelectic energy probe, D2 - high speed photodiode, Fl and F4 - UV blocking filter, F2 and F3 - UV pass filter, HC1 - uranium/neon hollow cathode lamp, 11 and 12 - beam contracting telescope, 13 - 25 em focal length lens, P -prism. Figure 2. Pulsed photoacoustic spectrum of a portion of the 1Al + A1 B2 transition in aniline obtained using a x 0.37 torr sample pressure. The photoacoustic signal has been corrected for the variation in incident laser pulse energy with wavelength. ranged from 1 to 10 ~J. Pulse energies The vibronic bands are v'" which implies a transition labeled by the symbol kv from v" quanta in mode k in the ground electronic state to an excited electronic state with v' quanta in mode k. I represents the NH 2 out-of-plane rocking mode. modes. 6a and 12 represent a 1 symmetry ring 1 represents a ring-stretching mode. assignment was taken from Ref. 14. The - 13 5- Figure 3. Two-color time-resolved spectra of aniline. PUMP pulse excites the s 0 + 34,032 em -1 The s 1 0-0 transition at The TRANSFER pulse photon energy is 18,797 cm-l (532 nm). The abscissa gives the time delay between pulses, ~t = tTRANSFER - tPUMP. Multiphoton ionization detection. pressure was 0.010 torr. energies were 0.5 ~J The sample PUMP and TRANSFER pulse and 2.0 mJ, respectively. (b) Photoacoustic detection. was 0.38 torr. (a) The sample pressure PUMP and TRANSFER pulse energies were approximately 5.4 ~J and 6.5 mJ, respectively. Both spectra have not been corrected for fluctuations in pulse energy. Figure 4. Energy level diagram which shows the excitation processes corresponding to the spectra of Fig. 3. The PUMP . and TRANSFER pulses have photon energies of 34,032 em -1 and 18,797 em -1 , respectively. Figure 5. Two-color time-resolved spectra of aniline. The PUMP pulse excites the s + s 1 0-0 transition at 0 1 34,032 cmThe TRANSFER pulse photon energy is 32,787 cm- 1 (305 nm). The abscissa gives the time delay between pulses, ~t = tTRANSFER - tPUMP" Multiphoton ionization detection. pressure was 0.004 torr. (a) The sample PUMP and TRANSFER pulse energies were 0.4 ~J and 13 ~J, respectively. (b) Photoacoustic detection. The sample pressure -136- was 0.37 torr. were 5.2 ~J PUMP and TRANSFER pulse energies and 300 ~J, respectively. The dashed line gives the expected noise-free signal. Both spectra have not been corrected for fluctuations in pulse energy. Figure 6. Two-color time-resolved photoacoustic spectrum of aniline. The conditions are similar to those of Fig. Sb except that a wider range of ~t is shown. The dashed line indicates the expected signal if noise is removed. The downward drift for ~t > 5 psec is discussed in Section 4 . 2. Figure 7. Energy level diagram which shows the excitation processes corresponding to the spectra of Fig. Sa, Sb and 6. The PUMP and TRANSFER pulses have photon energies of 34, 032 em tively. -1 and 32 787 em -1 , respec- The potential curves and vibrational levels do not correspond to known molecular parameters. Figure 8. Two-color time~resolved MPI spectrum of 0.004 torr sample of aniline. The 1.2 ~J PUMP pulses excites the s + s 1 0-0 transition at 34,032 cm- 1 • The 0 6.2 lJJ TRANSFER pulses have photon energies of 37590 cm-l (266 nm). The abscissa gives the time delay between pulses, At = tTRANSFER - t PUMP' The spectrum has not been corrected for fluctuations in pulse energies. -13 7Figure 9. Two - color time-resolved MPI spectrum of a 0.021 torr sample of benzene. -1 -1 The 0.6 ~J PUMP pulses ' excited the X A1 g + A B2u v 18 -o transition at 38607 em -1 • The 1.4 ~J TRANSFER pulses had photon energies of 37590 cm- 1 (266 nm). The abscissa g-ives the time delay between pulses, Llt - tTRANSFER tPUMP" The spectrum has not been corrected for fluctuations in pulse energies. -138- D1 DYE LASER #2 Nd:YAG LASER C1 Ll L2 --A HCL MPI OR PAS CELL D1 D2 F4 N2 LASER 1----~ DYE LASER #I Figure 1 - 139 - I() co 0 0 0 C\1 I() f() w z -z....J <{ 0 0 I() -0 0 0 (.0 (1) '=t IE C\1 f() E c: u ............ - "" ~ (1} ..c - ..c: Cl c: E (1} c: (1} > > (1} :J a ao ~0 0 ~ 0 I 0 '=t f() I() (1) C\1 Fi gure 2 -140- (a) (/) (b) ·c: ::> >- ~ 0 ~ :.0 ~ ~ E go.5 - -~ ( /) ::J 0 (.) 0 ~ c... Figure 3 -141- 70000 ~onization ~ 60000 Adiabatic IP -------------Pump 50000 ~at ion Transfer 40000 30000 20000 Pump 10000 0 Figure 4 Transfer -142- ANILINE (a) (/) :t:: c :::> ... ... :... a >- 0 ~ - 0.5 0 c .2' en (b) (/) 0-4~0~~--~-2~0--~--~0~--~--~2~0--~--~4~0--~--~60 D.t /nsec Figure 5 -143- ~------r---------------------~~ w z _.J z <l: 0 u (l) (/) - ~~ <J I '------ 0 Figure 6 - 144 - 70000 -Vertical IP Adiabatic I P 60000 50000 ION +e- Transfer Transfer 40000 30000 20000 Pump 10000 0 Figure 7 -145- w z -z_J <( uQ) "' 0~ - <l l.{) 0 Sl!un AJDJl!qJ'V/ 1ouo!s IdV'J Figure 8 0 -146- 8 <.0 w z w N z w CD (.) 0~ ~~ - <] 0 0 0 C}J § s~!un AJOJl!qJV'/ 1ouf>!S IdV'J Figure 9 0 -147- Paper IV: Detection of Weak Absorptions in the Visible and Ultraviolet by Pulsed Laser Photoacoustic Spectroscopy. -148- Detection of Weak Absorptions in the Visible and -~---~--------------------------------------------------- Ultraviolet by Pulsed Laser Photoacoustic Spectroscopy --------------------~-------------------------------------------- D. J. Mollt and A. Kuppermann The application of pulsed laser photoacoustic spectroscopy to the study of weak absorptions in the visible and ultraviolet spectral regions is discussed. The pulsed technique is compared to more conventional CW laser photoacoustic spectroscopy. Several examples of triplet state spectra are given. The sensitivity of the pulsed technique for extremely weak absorptions is limited by the increased probability of nonlinear effects when high intensities are used. tWork performed in partial fulfillment of the requirements for the PhD degree in Chemistry at the California Institute of Technology. -149- 1. Introduction A gas phase molecule in an excited state created by absorption of light can lose its energy and return to the ground state in the following ways: deexcite by emitting a photon. (1) It can radiatively (2) The absorbed energy can induce a photochemical reaction such as a rearrangement or bond cleavage process. (3) The molecule can transfer in- ternal energy to another molecule's internal degrees of freedom through inelastic collisions. (4) A collision with another molecul~ can convert internal energy into translational or kinetic energy of motion between the two molecules. The last process is the basis for most gas phase photoacoustic spectroscopy experiments. In typical experiments of this type, an intensity or wavelength modulated light beam passes through a gaseous sample and is partially absorbed. The amount of absorption is determined by the absorption cross section and concentration of the absorbing species. Some fraction of the deposited energy is converted into kinetic energy through collisional deactivation, resulting in a localized pressure increase which is detected by a nearby microphone as a photoacoustic signal.~ Processes (1) and (3) can decrease or delay the photoacoustic signal. Process (2) can either contribute to or decrease the photoacoustic signal depending on the nature of the system being studied. 1 - 3 -150- Photoacoustic spectroscopy was first reported by Bell in 1881. 4 Recently, due to the widespread availablity of laser sources, it has experienced a widespread utilization for the study of liquids and solids as well as gases. 5 - 7 Its primary advantages are high sensitivity, simplicity, applicability to solids and liquids as well as gases, and its ability to detect nonradiative decay processes rather than radiative processes. This paper presents an overview of our work on a particular application of photoacoustic spectroscopy --detection of weak absorptions in the visible and ultraviolet wavelength regions using pulsed laser excitation. The ad- vantages and disadvantages of pulsed excitation will be discussed and illustrated with spectroscopic examples. 2. CW vs. Pulsed Excitation Many recent photoacoustic studies have used a mechanical chopper to modulate the output of a CW laser in the 10 - 10 4 Hz range. The high average powers of these lasers and the sensitivity of the photoacoustic technique have allowed the . . t h e 10 - l 0 em -l . measurement o f a b sorpt1on coe ff.1c1ents 1n range using photoacoustic cells with a length of only 10 cm 8 . The weak absorptions due to forbidden electronic transitions, however, usually require excitation wavelengths in the blue or ultraviolet spectral regions. CW dye lasers have high - 151- average power outputs in the visible, but are subject to very low conversion efficiencies when their output is frequency doubled into the ultraviolet. Pulsed lasers have much higher conversion efficiencies for doubling to produce UV (up to 40%). A variety of nonlinear processes such as frequency doubling, frequency mixing, stimulated Raman shifting, 3rd harmonic generation, and other high order processes are now routinely used with pulsed lasers to produce tunable coherent output at wavelengths as short as 38 nm. 9 Photoacoustic spectroscopy with pulsed excitation has . numerous recent stu d"1es. 10-21 b een use d 1n Almost all studies to date, however, have not attempted to measure extremely small absorptions in the ultraviolet using tunable pulsed UV laser excitation. We have attempted to investigate spin- forbidden singlet-t!iplet absorptions using this technique. The sensitivity of this technique, as compared to conventional CW photoacoustic techniques, is influenced by several factors. First, the average power of most tunable pulsed UV lasers is considerably lower than CW visible dye lasers or co 2 lasers. This immediately decreases the photoacoustic signal because fewer photons are available to be absorbed. Some of this loss can be regained by gating the detection electronics so that only the amplitude of the first pressure wave emitted from the laser beam interaction region is detected. In effect, this concentrates the absorbed energy into an expanding -152 - cylindrical shell which moves out radially from the laser beam rather than distributing it uniformly throughout the cell as in the CW case. Therefore, for a given amount of input energy, the pulsed excitation produces a larger amplitude signal. Gated detection also allows almost total elimination of background signals due to light scattering or absorption at the windows·. By simply moving the windows back away from the microphone, the pressure increases from window effects arrive at the microphone later then the pressure wave caused by gas sample absorption. Gated de- tection, in general, allows a partial regaining of the sensitivity lost because of lower average power in the UV spectral region. Pulsed photoacoustic spectroscopy differs from the CW case in a second way. When CW excitation is employed with a low chopping freqpency (~10Hz), the excited molecules have a long time period during which they can collisionally relax before the light source is turned back on . Pulsed techniques derive their high sensitivities from pressure wave phenomena. If deexcitation is slow, >10 ~sec, a great reduction in sensitivity occurs because the pressure wave is distributed over a large spatial and temporal region, reducing its maximum amplitude. - For example, the o2b lL:+g state near 762.0 nm is known to collisionally deactivate with a time 22 CW intraconstant of about 3 X lo- 3 seconds at 300 torr. cavity photoacoustic spectroscopy detects absorption to this -153 - . h an exce 11 ent s1gna . 1 -to-no1se . s t ate w1t rat1o. 23 0 Our attempts to detect this state using pulsed photoacoustic detection failed because the long collisional relaxation time prevented the formation of pressure waves. The CW technique is much more sensitive to absorption in molecules with long collisional deactivation times than the pulsed technique, making the latter a less universal detection method. A third factor in favor of CW photoacoustic detection arises from the typical cavity design of CW lasers. Because of the high threshold for lasing and the low peak powers (compared to pulsed lasers) most output coupling mirrors for CW lasers transmit only 1 to 10% of the incident light. Output coupling for pulsed lasers can be as high as 96%. This causes the circulating power inside the cavity of a CW laser to be much larger than the actual output power. Placing a photoacoustic cell inside the cavity of a CW laser produces effective incident power levels of 80 watts or higher. 23 Fourth, pulsed excitation offers significant advantages because of the possibility of performing time-resolved studies using two laser pulses. This technique has been demonstrated in solids 11 and used also in gas phase studies of p - difluorob enzene 24 an d an1"1"1ne. 3 The high intensities involved in pulsed excitation pro duce a fifth major difference between pulsed and CW techniques, -154nonlinear processes. The signals produced by multiphoton absorption can surpass the expected signals from weak spinforbidden transitions when high intensities are employed. This effect will be discussed in more detail in Section 4.4. 3. Experimental Photoacoustic cells for pulsed excitation should be designed in such a way that the amplitude of the detected pressure wave caused by gas absorption is maximized, and the background signals caused by window scattering and window absorption are minimized. 3.1 Photoacoustic Cell We have tried a number of different cell designs. The most recent model gives high sensitivity, good reproducibility . 1nexpens1ve . . . 1e to construct. 3,24 It consists an d 1s an d s1rnp 0 of a 18 ern long by 0.9 ern inner diameter cell constructed of stainless steel and aluminum. The central portion of the cell is constructed from a 1/2" Swagelok tee and stainless steel tubing. Quartz windows are mounted at each end using Viton a-ring seals. A Knowles Electronics BT17S9 25 miniature electret microphone with a nominal sensitivity of 0.01 Volt/ Pascal is positioned over a 3/16" diameter hole in the side of the tee and held in place with epoxy resin. To maximize the signal from the pressure wave, the side of the tee was ~ 155- milled down before mounting the microphone to allow the microphone to be as close as possible to the laser beam. The cell length was chosen to delay the signal caused by scattered light and absorption at the windows, which results from heating of the cell walls or windows, respectively. Scattered light signals are further reduced by placing four apertures inside the cell and an aperture outside the cell at its entrance. 3.2 Electronics The electret microphone used in these studies has a built-in FET preamplifier which is powered by a 1.5 volt battery. The preamplified signal was typically limited to a frequency range of 10 2 - 10 4 Hz by an electronic filter and amplified. The peak of the first pressure wave was integrated by a differential gated integrator system with an integration window of 15 ~sec. Signals from a pyre- electric joulemeter were processed in a similar manner by a second integrator system to provide a normalization signal. A microcomputer system controlled the dye laser grating scan, doubling crystal phasematching angle, and digitized 0 and stored signals from both integrators every 0.1 A. 3.3 Laser Systems The spectra presented in this paper were obtained using two different dye laser systems. A Molectron UV400 N2 laser ,.15 6,- was used to pump a Molectron DL200 dye laser. The dye laser fundamental output down to 360 nm was used. Pulse energies ranged from 1 to 250 ~J/pulse with a ~6 x 10- 9 sec pulsewidth, a linewidth of about 0.5 cm- 1 , and a 20Hz repetition rate. For weaker absorptions, a more powerful laser system was used, consisting of a Molectron MY34 Nd:YAG laser which pumped a DL18 dye laser. Visible output, obtained from various dyes pumped by the 2nd and 3rd harmonics of the Nd:YAG laser, ranged from 1 tO SO mJ/pulse with a linewidth of about 0.5 cm- 1 at a 10 Hz repetition rate. UV output was produced by doubling the dye laser output or by mixing the dye laser output with the 1064 nm Nd:YAG fundamental. During wavelength scans, the KDP crystal phasematching angle was adjusted every 0 0.1 A by a microcomputer using a predetermined 5th order polynomial in wavelength. UV outputs ranged from 1 to 10 mJ/pulse. All spectra were normalized with respect to incident pulse energy to correct for the wavelength dependence of the dye gain curve and output pulse energy fluctuations. 4. Results This section presents specific examples of spectra ob- tained using the pulsed laser photoacoustic technique which demonstrate the sensitivity, usefulness and limitations of this method. -157~ 4.1 High Vibrational Overtones Figures 1 and 2 show pulsed laser photoacoustic spectra of some high vibrational overtones in acetylene. Vibrational overtones of acetylene 26 as well as numerous other polyatomic molecules have been studied by the CW photoacoustic technique. In Fig. 1, the strong band system centered near 15,600 cm- 1 has been assigned as the v a 5 C-H stretch overtone. 26 Another weaker band can be seen centered at 15,530 cm- 1 . In all, five vibrational bands have been detected in this region using a high resolution intracavity CW laser photoacoustic technique where the wavelength of the dye laser is modulated slightly instead of the usual intensity modulation technique. 26 The greater sensitivity at visible wavelengths of the CW intracavity technique, compared to the pulsed technique, is demonstrated by this example. This same band system was also 27 studied previous by . long path absorption. Figure 2 shows a pulsed photoacoustic spectrum of the v a 7 CH stretching overtone in acetylene centered at 21,210 nm. This preliminary spectrum, although noisy, still displays the characteristic P and R branches and is the first reported spectrum of this particular state. With additional signal averaging, the individual J lines could easily be resolved, permitting a rotational analysis. We have also investigated high vibrational overtones of cn 3H in the regions of the v 2 5, 6 and 7 levels of the C-H stretch mode using the pulsed photoacoustic spectroscopy . tee h n~que. 28 -158- 4. 2 Spin-Forbidden Transitions with Spinorbit Coupling In the usual level of approximation, optical transitions between states of different spin multiplicity are not allowed. In all molecules, however, if the level of approximation is increased, this transition becomes possible, but remains weak. Spin orbit coupling, which couples the electron spin and angular momenta, can perturb the zero order states by mixing in character of states with different spin multiplicities. 29 This perturbation becomes larger in nuclei with higher charges, producing transitions of slightly more intensity between levels of different multiplicity than for molecules with light nuclei. Oscillator strength is a convenient criterion for classifying the strengths of electronic transitions since it is propertional to the integral of the absorption coefficient over the entire electronic band. 29 Spin-allowed transitions typically have oscillator strengths falling between 1 and 10 -4 , de- pending on symmetry selection rules and dipole transition moments. Spin-forbidden transitions have oscillator strengths falling between 10- 5 and lo- 10 , depending on the symmetry requirements for spin-orbit coupling and the presence of heavy nuclei. We have attempted to detect spin-forbidden transitions which lie in the 10- 5 - 10- 6 range in molecules which contain oxygen, sulfur or halogen atoms as spin orbit enhancer~, and also transitions which are near the 10- 10 limit in hydrocarbons, using the pulsed photoacoustic technique. -15 9- CS 2 . The absorption bands in CS 2 from 390 to 330 nm . 30-38 have been studied extens1vely. The vibrational and rotational structure in the absorption spectrum indicates that the spectrum results from transitions between a linear ground state ( 1 L;) and a bent excited state of B2 syrnrnetry. 32 Zeeman effects indicate that the observed B2 state is a com33 ' 34 . 1 et sp 1 1tt1ng. . . . h su b stant1a . 1 tr1p ponent o £ a 3A2 state w1t Variable-angle low-energy electron impact spectroscopy also 37 confirms the singlet-triplet nature of the transition. Figure 3 shows a pulsed photoacoustic spectrum of cs 2 obtained using a maximum energy/pulse of 110 ~J and a sample pressure of 300 torr. Barrow and Dixon 35 have determined that the oscillator strength of the a3A2 state in cs 2 is about 5 x 10- 7 , placing it among the stronger spin-forbidden transitions, as expected from the presence of sulfur atoms which enhance the spin orbit coupling. The spectrum of Fig. 3 has an excellent signal-to-noise ratio, which implies that this technique should be applicable to the study of much weaker transitions. so 2 • The role of so 2 in atmospheric chemistry 39 has stimulated considerable interest in its spectroscopy. The . 1· n so (X- 1A ~ a- 3 B ) lowest energy triplet state a b sorpt1on 1 1 ~ 2 is centered at 370 nm. This state has been investigated by 40-42 phosphorescence 43 and laser-induced phos. absorpt1on, phorescence.44 -160- Figure 4 shows a pulsed photoacoustic spectrum of so 2 which includes well-resolved transitions to the lower vibrational levels of the 3B1 state. Clearly resolved rotational structure is present in the spectrum of the (OOO)~ (110) transition shown in Fig. 5. The rotational perturbation observed by Brand and co-workers 42 and Merer 40 at the K' = 15 level and upward near 368 nm is also present in the photqacoustic spectrum of Fig. 5. This perturbation is believed to arise from an interaction with b 2 vibrational levels of a nearly 3A2 state. Biacetyl. The interactions between the low-lying singlet and triplet states of biacetyl have received extensive study . 1u d.1ng fl uorescence, 45-48 by a variety of techniques 1nc . 51 ' 52 and CW photoabsorption, 49 , 50 molecular beam techn1ques acoustic spectroscopy. 53 Energy transfer into the biacetyl 3 T1 C Au) state from triplet states in other molecules has often been used to study nonradiative processes in these molecules. 53 , 54 Figure 6 shows a pulsed photoacoustic spectrum of the T1 ( 3Au) state of biacetyl. In contrast to cs 2 and so 2 , the biacetyl triplet spectrum contains broad unresolved features. Vibronic bands appear only as weak peaks superimposed on a broad electronic band Thiophosgene. The visible absorption system in thio- phosgene (CSC1 ) consists primarily of two overlapping weak 2 electronic bands . The lowest energy band has been assigned ~ a-3A 2 (n ~ rr *) trans1t1on . . 55-58 wh 1c . h h as an -161- origin at 17492 em -1 The higher energy band is a symmetry forbidden X1A1 + A1A2 (n + TI*) transition 59 with an origin near 18730 cm- 1 • Thiophosgene belongs to the small class of molecules in which fluorescence has been detected from the second excited singlet state, 60 , 61 in violation of Kasha's rule that polyatomic molecules emit only from the lowest electronically excited level of a given multiplicity. 62 A pulsed photoacoustic spectrum of the weak visible absorption system of thiophosgene is shown in Fig. 7. This spectrum is a composite spectrum formed by combining several spectra taken using different laser dyes. The spectral region surrounding the origin of the a3A2 state is shown in Fig. 8. The wide dynamic range of the pulsed photoacoustic technique is apparent from a comparison of Figs. 7 and 8. The examples given in this section of pulsed photoacoustic spectroscopy of triplet states demonstrate the usefulness of this technique for studying relatively strong spin-forbidden transitions. Unfortunately, all triplet states given as examples have been studied previously by long pathlength absorption experiments. The photoacoustic studies provide no additional spectroscopic information. They simply demon- strate that the pulsed photoacoustic technique is capable of detecting these transitions. Weaker transitions which fall in the lower range of oscillator strengths for spin-forbidden transitions are not detected easily by this method . or the -162long pathlength absorption technique . The problems which arise when studying very weak transitions will be discussed in the following sections. 4.3 Overlapping Bands In most molecules, only low-lying triplet states have been studied spectroscopically. Broad singlet states often mask the weak absorptions arising from triplet states at higher energies. 29 The photoacoustic technique is also not immune to this problem. Figure 9 shows a spectrum of 1 , 1 - dichloroethylene in the region where the lowest triplet state is expected to appear. 63 , 64 Our pulsed photoacoustic spectros copy attempt to observe this state in ethylene failed because of nonlinear effects similar to those described for benzene in the next section. We hoped that the chlorine substituents would increase the ~riplet state trans i tion intensity through enhanced spinorbit coupling. Unfortunately, as shown in Fig. 9, a strong signal is observed which rises rapidly with in creasing photon energy. This signal results from the tail of an absorption due to a strong charge transfer band involving the chlorine substituents. The expected triplet state spec- trum is obscured by this charge transfer band . 4.4 Nonlinear Effects At high intensities the probability of nonlinear optical effects such as multiphoton absorption, stimulated Raman -163- scattering, frequency doubling, etc. increases dramatically with respect to ordinary linear absorption. The major ad- vantage of the pulsed photoacoustic technique is that it allows the use of pulsed lasers with high intensity outputs which permit efficient generation of tunable UV output using nonlinear optical processes. The high intensities required for efficient UV generation and higher sensitivity place an upper limit on the sensitivity of this technique. As inten- sity increases, the probability of multiphoton absorption increases, and eventually surpasses the probability of absorbing only one photon in an extremely weak spin-forbidden transition. When this happens, the photoacoustic spectrum is produced by states at two, three or four times the photon energy, instead of the one-photon resonance with the triplet state. Nonlinear absorption predominated when we attempted to -lA lg + -3B . . 65 1n . b enzene us1ng . stu d y t h e X a lu trans1t1on t he pulsed photoacoustic technique. This extremely weak transition has been observed in liquid 66 - 71 and gaseous 72 , 73 benzene 0 from 3000 to 3400 A in the presence of 0 2 . It has not been observed in the free molecule. Figure 10 shows a pulsed photoacoustic spectrum in the region 330 to 322 nm. Three broad peaks are present which do not resemble the expected triplet state spectrum. Similar peaks do appear, however, in absorption spectra in the vacuum - 164 UV region at twice the photon energy. 74 Closer examination indicates close agreement between the positions of these states and the Rydberg states identified as 2R 020 , 2R and 040 2R 000 byWilkinson. 74 In our spectrum, however, the trans itions are shifted to higher energy by about 380 cm- 1 . The strength of the Rydberg transitions in ben zene led Wilkinson to assign them as 1 1 1 A1 g + E1u or Azu allowed transitions. The presence of inversion symmetry in benzene suggests that vibronic coupling with a u - type vibrational mode is required to see these same states in a two -photon spectrum. The v 20 Ce 2u) mode with a ground electronic state frequency of 398 cm - l may be active in the vibronic coupling, since it also appears in vibrational progressions involving other nearby Rydberg states in benzene. 74 Vibronic coupling with the v 20 mode would explain the 380 cm- 1 shift in energy of the observed states between the one- and two-photon spectra. Further information of the two-photon nature of the pulsed photoacoustic spectrum of benzene is provided by Fig. 11 which shows a rnultiphoton ionization (MPI) spectrurn 38 of benzene in the same spectral region. MPI spectroscopy is known to be extremely sensitive to resonances with Rydberg states . The transitions present in the photoacoustic spectrum also appear in the MPI spectrum with similar intensities and wavelengths. The MPI signal was roughly proportional to intensity squared. -165- Similar nonlinear absorption effects appeared in pulsed photoacoustic spectra of 1,3-butadiene and hexatriene when we tried to detect extremely weak spin-forbidden transitions in those molecules . In the benzene experiments, this incident beam was unfocused with a diameter of ~s mm. Expanding the beam diameter would decrease nonlinear absorption. It also appears to sub- tantially decrease the pulsed photoacoustic signal. Apparently, the absorbed energy is distributed over a larger volume. This decreases the maximum amplitude of the detected pressure wave. In benzene we were unable to decrease the two - photon signal enough to allow observation of the triplet state spectrum. 5. Conclusions The results of our triplet state studies indicate that the pulsed photoacoustic technique is easily able to detect spin-forbidden transitions which have a significant amount of spinorbit coupling. Triplet states in molecules of this type, however, have been studied extensively using long pathlength absorption techniques. Extremely weak spin-forbidden trans- itions in hydrocarbons are difficult to detect by pulsed photoacoustic methods because of the increased probability of nonlinear absorption to higher states in these molecules. The nonlinear absorption can easily be larger than the absorption due to the one-photon spin-forbidden transition. The development of tunable CW lasers with high power UV outputs -166- or high power long pulse tunable UV lasers should minimize nonlinear effects. These lasers, when combined with present photoacoustic detection techniques, should provide an effective method for studying extremely weak absorptions in the ultraviolet. The recently demonstrated two-color time-resolved pulsed . tee hn1que . 24,28 prov1"d es a new met h o d o f measur1ng . p h otoacoust1c excited state absorption and dynamics. The sensitivity of the pulsed photoacoustic technique should allow these studies to be performed using triplet states with significant amounts of spinorbit coupling as intermediate states. -167References 1. W. R. Harshbarger and M. B. Robin, Ace. Chern. Res. ~' 329 (1973). 2. W. R. Harshbarger and M. B. Robin, Chern. Phys. Lett. ~' 462 (1973). 3. D. J. Moll, G. R. Parker, Jr. and A. Kuppermann, "TwoColor Time-Resolved Photoacoustic and Multiphoton Ionization Spectroscopy of Aniline", to be published. 4. A. G. Bell, Philos. Mag. 11,510 (1981). 5. Y.-H. Pao, edit., Optoacoustic Spectroscopy and Detection (Academic Press, New York, New York, 1979). 6. A. Rosencwaig, Photoacoustics and Photoacoustic Spectres~ 7. (Wiley, New York, 1980). C. K. N. Patel and A. C. Tam, Rev. of Mod. Phys. ~' 517 (1981). 8. C. K. N. Patel .and R. J. Kerl, Appl. Phys. Lett. 30, 578 (1977). 9. J. Reintjes, C.-Y. She and R. C. Eckardt, IEEE J. Quant. Elect. QE-14, 581 (1978). 10. T. F. Deutsch, Opt. Lett.!, 25 (1977). 11. M. G. Rockley and J. P. Devlin, Appl. Phys. Lett. l!, 24 (1977). 12. J. G. Parker and D. N. Ritke, J. Chern. Phys. 59, 3713 (1973). 13. R. W. Shaw, Appl. Phys. Lett. ~' 253 (1979). - 16 8- 14. J. E. Allen, Jr., W. R. Anderson and D. R. Crosley, Opt. Lett. 15. !, 118 (1977). T. Aoki and M. Katayama, Jap. J. App1. Phys. !Q, 1303 (1971). 16. P. C. Claspy, C. Ha and Y.-H. Pao, Appl. Opt. !£, 2972 (1977). 17. C. W. Brue, B. Z. Sojka, B. G. Hurd, W. R. Watkins, K. 0. White and Z. Derzko, Appl. Opt. ~, 2970 18. G. A. West and J. J. Barrett, Opt. Lett.!, 395 (1979). 19. D. A. Siebert, G. A. West and J. J. Barrett, Appl. Opt • .!2_, 53 (1980). 20. G. A. West, D. R. Siebert and J. J. Barrett, J. Appl. Phys. 21. ~' 2823 (1980). S. L. Chin, D. K. Evens, R. D. McAlpine and W. N. Selander, Appl. Opt. 21, 65 (1982). 22. S. A. Lawton,.S. E. Novick, H. P. Broida and A. V. Phelps, J. Chern. Phys. 66, 1381 (1977). 23. K. V. Reddy, PhD Thesis, University of Wisconsin-Madison (1977). 24. D. J. Moll, G. R. Parker, Jr. and A. Kuppermann, "Photoacoustic Detection of Stimulated Emission Pumping in p-Difluorobenzene", to be published. 25. Knowles Electronics, Inc., 3100 N. Mannheim Rd., Franklin Park, IL 60131, USA. 26. G. J. Scherer, K. K. Lehmann and W. Klemperer, "The HighResolution Visible Overtone Spectrum of Acetylene", in press. -169- 27. K. Hedfeld and P. Lueg, Zeitschrift Physik 77, 446 (1932). 28. D. J. Moll, J. W. Perry, A. Kuppermann and A. H. Zewail, "High Overtone Photoacoustic Spectroscopy of Some MethylContaining Molecules. I. Rotational Analysis", to be published. 29. S. P. McGlynn, T. Azumi and M. Kinoshita, Molecular Spectroscopy of the Triplet State, (Prentice-Hall, New Jersey, 1969). 30. L. N. Liebermann, Phys. Rev. 60, 496 (1941). 31. A. E. Douglas, Can. J. Phys. 36, 14 7 (1958). 32. B. Kleman, Can. J. Phys. 41, 2034 (1963). 33. A. E. Douglas and E. R. v. Hilton, J. Chern. Phys. 41, 357 (1964). 34. J. T. Hougen, J. Chern. Phys. 41, 363 (1964). 35. T. Barrow and R.N. Dixon, Mol. Phys. 36. M.-J. Hubin-Franskin and J. E. Collin, J. Electron Spectrosc. 37. 2, 139 (1975). ~' 3910 (1978). R. Rianda, D. J. Moll and A. Kuppermann, Chern. Phys. Lett. 39. 137 (1973). W. M. Flicker, 0. A. Mosher and A. Kuppermann, J. Chern. Phys. 38. ~' Zl' 469 (1980). J. Heicklen, Atmospheric Chemistry (Academic Press, New York, 1976), p. 331. ~' 40. A. J. Merer, Disc. Faraday Soc. 41. J. C. D. Brand, V. T. Jones and C. Di Lauro, J. Mol. Spectrosc. 40, 616 (1971). 127 (1963). -17 0- 42. J. C. D. Brand, V. T. Jones and C. Di Laruo, J. Mol. Spectrosc. 45, 404 (1973). 43. A. G. Gaydon, Proc. Roy. Society 146A, 901 (1934). 44. F. B. Wampler, R. C. Oldenborg and W. W. Rice, J. Appl. Phys. 45. ~' 6117 (1979). H. Okabe and W. A. Noyes, Jr., J. Amer. Chern. Soc. ~, 801 (1957). 46. R. VanderWerf, D. Zevenhuijzen and J. Kommandeur, Chern. Phys. Lett. 47. ~' 325 (1974). G. M. McClelland and J. T. Yardley, J. Chern. Phys. 58, 4368 (1973). 48. C. S. Parmenter and H. M. Poland, J. Chern. Phys. 51, 1551 (1969). 49. J. W. Sidman and D. S. McClure, J. Amer. Chern. Soc. 22, 6461 (1955). SO. J. W. Sidman and D. S. McClure, J. Amer. Chern. Soc. 77, 6471 (1955). 51. M. Gurnick, J. Chaiken, T. Benson and J. D. McDonald, J. Chern. Phys. 74, 99 (1981). 52. J. Chaiken, M. Gurnick and J. D. McDonald, J. Chern. Phys . .z.i, 106 (1981). 53. M. B. Robin, J. Luminescence 12/13, 131 (1976). 54. K. Kaya, W. R. Harshbarger and M. B. Robin, J. Chern. Phys. ~' 4231 (1974). 55. L. Burnelle, J. Chern. Phys. 24, 620 (1956). 56. D. C. Moule and C. R. Subramanian, J. Mol. Spectrosc. !!, 336 (1973). -17158. W. M. Flicker, 0. A. Mosher and A. Kuppermann, Chern. Phys. 59. Lett.~' 183 (1978). J. C. D. Brand, J. H. Callomon, D. C. Moule, T. Tyrrell and T. H. Goodwin, Trans. Faraday Soc. 60. S. Z. Levine, A. R. Knight and R. P. Steer, Chern. Phys. Lett. 61. £!, 2365 (1965). ~' 73 (1974). D. Phillips and R. P. Steer, J. Chern. Phys. ~' 4780 (1977). ~' 62. M. Kasha, Disc. Faraday Soc. 63. A. Kuppermann, W. M. Flicker and 0. A. Mosher, Chern. Rev. 64. ~' 14 (1950). 77 (1979). D. G. Wilden and J. Comer, J. Phys. B: Atom. Molec. Phys. g, 1371 (1979). 65. A. C. Albrecht, J. Chern. Phys. 66. A. L. Sklar, J. Chern. Phys. 67. G. N. Lewis and M. Kasha, J. Amer. Chern. Soc. 66, 2100 ~' 354 (1963). i, 669 (1937). (1944). 6 8. 69. 70. 71. 72. 73. G. E. Kimball, J. Chem. Phys. 74. P. G. Wilkinson, Can. J. Phys. 34, 596 (1956). ~' 188 (1940). -172 Figure Captions Figure 1. Pulsed laser photoacoustic spectrum of acetylene in the region of v = 5 CH stretch overtone. sample pressure was 760 torr. The This spectrum and all subsequent spectra have been corrected for the variation of pulse energy with wavelength. Figure 2 . Pulsed laser photoacoustic spectrum of acetylene in the region of the v = 7 CH stretch overtone. The sample pressure was 1870 torr. Rotational lines are barely visible above the noise. The large con- stant background is produced by scattered light striking the walls of the photoacoustic cell. Figure 3. "'1 + Photoacoustic spectrum of the X rg ~ a3A2 transition of cs obtained using a sample pressure of 300 torr. 2 The band assignments at the top are from Kleman 32 where r, II, 2, •• •• ~ !)., ••• , correspond to k' = ~" = 0, 1, k is the symmetric top quantum number and is the quantum number associated with the vibra- tional angular momentum. The symbol 1,2 I 3 implies Vi = 1, Vz = 2, Vz = 3 and V3 = Vl = V3 = 0. Vibrational modes 1, 2 and 3 correspond to the symmetric stretch , bend and antisymmetric stretch with double prime and single prime corresponding to ground and excited electronic states, respectively . - 173 Figure 4 . Pulsed photoacoustic spectrum of a portion of the X1A1 + a3B1 transition in S0 2 . The sample pressure was 930 torr and laser pulse energies were ~so ~J. The vibronic bands are labeled with the symbol (ijk) where.i, j and k are the number of quanta in the excited state symmetric stretch, bend and antisymmetric stretch, respectively. Figure 5. Pulsed photoacoustic spectrum of the (110) vibronic band of the so 2 1A1 + a3 B1 transition which shows partially resolved rotational structure. The sample x pressure was 390 torr . Figure 6 . -1 Pulsed photoacoustic spectrum of the X Ag + ..a 3A u transition in biacetyl. ~20 Figure 7. The sample pressure was torr and pulse energies were about 200 ~J. Pulsed photoacoustic spectrum of the overlapping a3A2 and A~A states in thiophosgene. The sample pressure was 100 torr 6 The spectrum shown is a 2 composite of several spectra taken over different dye ranges. The relative amplitudes of the different spectra were adjusted in the regions of overlap. Figure 8. Pulsed photoacoustic spectrum of thiophosgene in the spectral region surrounding the origin of the X1A1 + a3A2 transition . The origin appears at 17,492 cm- 1 . The sample pressure was 100 torr. -174Figure 9 . Pulsed photoacoustic spectrum of 1,1-dichloro ethylene in the wavelength region where the lowest triplet state should appear. The observed signal results from the rising edge of a strong charge transfer band. The sample consisted of 200 torr dichloroethylene and 200 torr N2 to enhance the signal. Figure 10. Pulsed photoacoustic spectrum of benzene in the 1 Alg + a3Blu transition spectral region where the x should occur . The three broad peaks correlate with the 2R 020 , 2R 040 and 2R 000 Rydberg states studied by Wilkinson 74 if vibronic coupling with the v 20 Ce 2u) mode is invoked . The appearance of Rydberg state occurs through a two-photon absorption process. The sample pressure was 60 torr. Figure. 11: Multiphoton ionization spectrum of benzene in the same spectral region as Fig. 10. was 10 torr . The sample pressure The Rydberg transitions present in the spectrum of Fig . 10 also appear in this spectrum. >. - a.. ..c 0 0 0 - u 0 ::J ( /) u ~ ·- (f) 0 6~5~0--~~~--~--~~--~~~---L--~--~L---L---~---L--~--~ 646 648 644 642 640 636 638 Wavelength/ nm V1 " 0.5 Ol (1) "1 ·- 0 I ~ c ()q t:: "- <t '- ·..0- '- - '0 C2H2 15700 ..... "''1 - c :::::> (/) 1.0 15400 Wavenumber / cm- 1 15500 15600 -176- 0 ~~--------------------------------------~ N C\1 I u 7 C\1 1'- E ~ ~ ~ ...c - lo- Q) ....0 0 01 0 E ::J N cQ) E cQ) N N > 1'~ 0 s Figure 2 Q) > 0 5 c +- Cl) c: (/) +- ~ - 0 <! 5 ~ !..,_ _() ·- +- !..,_ 0 ~ (;:: QQ "''1 1-'• !..,_ ~ => c: +- (/) - 10 )~ 360 0,80,80,51,4 ~2 L2 LO lll II 3.44 I I I I I I 368 0,60,60,7 ~2 L:2 ¢3 3.38 0,70,70,80,4 0,80,51,4 ~2 L2 ¢3 LO n3 lll L2 - ~-~----.---- 3.40 362 364 366 Photon Wave length (nm) 1,51,5 ~2 L2 T--~---r 3.42 Photon Energy (eV) I I ""-l --l ~ -178- Wavenumber /em -I (000) (j') 27000 27500 (020) (I 00)(030) (110) (120)(200) 26500 26000 (010) 1.0 ·c ::J >- lo... 0 lo... :..0 lo... ~ 0 c 0' U; 0.5 u .(i) :::J 0 u 0 0 0 ...c 0.... 0~--~-M~--~~~--~------~~~~----~ 390 380 370 Wavelength/nm Figure 4 360 -17 9- <!) r<) ~-------------------------------------------------<!> 0 C\J Cf) 0 0 C\J 1'C\J E I E u 0 ~ 0 <1) .J:) 0 c > 0 C\J 1'r<) ~ ..c - 0' c ~ <1) > 0 E ;:, 1'- 0 3: <1) 3: 0 0 CD <!) C\J q I{) d s~!un AJOJ~!qJ'if/IDUD!S :)!~sno:)oo~o~d Figure 5 -180- _J >- 0 0 I- 0 u 0 N 0 0 w 1.() <! CI) I E ~ E ~ "'- 0 Q) ...0 1.()~ 0 o -Ol 1.() CD cQ) E <n ::J c Q) > ~ > Q) ~ 0 0 0 Ul <.0 (J) 1.() 0 S~!un AJOJ~!qJ'<;j/IDUD!S :)!~SnO::l00~04d Figure 6 0 -181- 0 en l() _N "'" u u TE E (.,) c: ""-o 0 ::::::J - Q) Q) 0 0 o ' O.s= """ 0 Q) l() ..c 0C\J Ol E c: Q) c: > > 3: 3: 0 0 0 CD Figure 7 Ol ~ t-j 00 (1) Q.. ..c. 0 -+- 0 0 u 0 ::l ( /) ·-u 584 580 576 572 Wavelength/ nm 568 564 CI 2 CS 17800 560 0 ~---~------~---~------~---~------~---~------~---~------._---~---~ 0.5 - ·(/) c -0 f-1· OQ 'T1 .......... <( ~ ...c ~ 0 - ~ ~ c::::> ~ 1.0 17200 Wavenumber / cm- 1 17400 17600 I I N I-' 00 -183- ~----------------------------------------------~~ C\J w w _j z >- 0 0 C\J tO j"(') ~ I I- C\J w 0 0::: 0_j I u 0 I 1'1'C\J 'E ~ lo.. <l) .c E ~ - ..c 0'1 c: E ::J <l) Q) c: > 8 0 ~ tO > <l) 0 co j"(') 1'C\J 0 0 co 1.{') j"(') Figure 9 3:• 0 ..... (!) >i ~ ()q ..... 'Tl - a.. .c. 0 0 0 - u 0 ::J r I tJ' . ,~ ..A j"\_ BENZENE 31000 I 0--------------~------~------~------~------~------~-----330 328 322 326 324 Wavelength/ nm ~0.5 u ·- (f) c Ol ·- 0 - ""'-..... <( ~ ·..0- - c ::::> (/) 30400 Wavenumber/ cm- 1 30600 30800 ~ 00 I ...... -185 N N r() 0 0 0 !'() w w z N z w m qN r() 0 0 co 0 !'() 'E E (.) c: ........... - ........... """ Cl> <.0 ~ N E ::;, r() ~ 0"1 c: Cl> c: Cl> >o oo Cl> > 0 s 3:w 0 !'() 0 0 v 0 !'() Figure 11 -186- APPENDIX A: A Description of Laser Systems and Related Electronics. -1871. Introduction The lasers used in the experiments described by Papers I-IV in the main body of this thesis consisted of a Molectron MY34 pulsed Nd:YAG laser, a Molectron DL18 dye laser, a Molectron UV400 pulsed N2 laser, and a Molectron DL200 dye laser. These lasers and their operation are described in detail in the Molectron service and operating manuals supplied with each laser. The specific details of operating each laser will not be covered here. Instead, I will describe briefly the general features of each laser. The crystal scanning drive on the DL18 dye laser was interfaced to the microcomputer used to drive the grating in the DL200 laser and collect data. The microcomputer and the interfacing of the grating scan system were described in the thesis of Ronald Rianda and will not appear here. The details of the crystal scanning circuitry will be described in Section 3. Papers II and III contain the results of two-color time-resolved experiments. The circuitry involved in triggering the Nd:YAG and N2 lasers will be described in Section 4. 2. Laser Systems 2. 1 MY34 Nd:YAG Laser Figure 1 shows an optical schematic of the MY34 Nd:YAG laser. The MY34 uses a proprietary cavity design which -188- combines an unstable resonator with polarization output coupling . The unstable resonator design allows the beam to fill the active volume of the Nd:YAG rod. The polarization extraction produces a low-divergence, near diffractionlimited beam with a near-Gaussian intensity profile. The polarizer in the cavity transmits p-polarized light and reflects s-polarized light. Therefore, light in the cavity traveling to the right in Fig. 1 is p-polarized. When this light returns, after passing through the A/4 plate twice and reflecting off M2 , it is s-polarized and gets extracted from the cavity. The Pockels cell is a cylindrical-ring- electrode KD*P Pockels cell which fires approximately 150 ~sec after the flashlamp trigger signal. Light which leaves the oscillator is amplified by a second Nd:YAG rod/flashlamp system an~ directed into the harmonics box. The Nd:YAG fundamental is doubled using a type II KD*P crystal in a temperature-controlled oven. Another similar crystal mixes the fundamental and second harmonic to produce the third harmonic at 355 nm. A versatile series of dielectric beamsplitters and total reflectors allows any beam to be directed to any output port. The specified output pulse energy for each harmonic is shown below Wavelength Pulse Energy 1064 nm 532 355 700 mJ 225 125 -189In April 1980, when the laser was received, the 532 nm output pulse energy was 360 mJ. The laser fires at a 10 Hz repetition rate with an adjustable pulse length of 15-30 nanoseconds for the fundamental. 2.2 DL18 Dye Laser The DL18 dye laser is shown schematically in Fig. 2. The second or third harmonic of the MY34 Nd:YAG laser is directed into an oscillator dye cell and one or two additional amplifier dye cells, depending on the operating configuration, using dielectric beam splitters or total reflectors. The oscillator consists of a grating, output coupling mirror, 40X achromatic prism beam expander, halfwaveplate, and dye cell. Typically, 10-20% of the pump beam is directed into the oscillator dye cell. pump beam is direct~d The remaining into one or two amplifier dye cells. The second amplifier, the post amplifier, can be transversely or longitudinally pumped. For longitudinal pumping, the pump beam is decoupled from the dye laser output by pumping in a counter-propagating, slightly off-axis direction, and the oscillator output passes through a 4X beam expander to match beam diameters in the post amplifier. The DL18 dye laser output has a specified linewidth of 0.01 nm at 560 nm and conversion efficiencies of 30% and 20% at 560 nm and 500 nm, respectively. Using different dyes, the fundamental - 190scanning range is 400-900 nm. Table I contains a list of laser dyes and suggested starting concentrations for the DL18 dye lser. The wavelength range is extended by using nonlinear frequency doubling and mixing . KPB, KDP and KD*P crystals in temperature-controlled ovens can be mounted in a rotatable mount to allow angle phasematching . The KDP and KD*P crystals (D, E, F and G) use type I phasematching. Pump beams enter the crystals polarized in a vertical plane. The UV output is horizontally polarized. Phasematching is achieved by ro- tating these crystals about a vertical axis. The KPB crystals (B and C) also use type I phasematching with vertically polarized pump and hori zontally polarized output beams. These crystals rotate around a hori zontal axis for phasematching. For frequency doubling, the crystal with the appropriate wavelength range is mounted in the dye laser output beam. When the phasematching angle is correct, the UV output is detected easily from the fluorescence it produces when it strikes a piece of white paper . The residual fundamental beam is eliminated by UV pass/visible blocking filters or the dispersive prism system described in the section on stimulated Raman scattering. During wavelength scans the phasematching angle is adjusted by a micro-computer which also collects data and scans the grating, as described in the next section . Oscillatorb 8.3 X 10- 4 5.0 X 10- 3 4.5 X 10- 3 1.2 X 10- 2 3.0 X 10- 4 5.0 X 10- 4 1.5 X 10- 3 <10-3e <10- 2 1.0 X 10- 2 d " 1st Amplifier 1.0 x 10- 3 Not Determined -3 3.3 X 10 -3 6.0 X 10 I Post Amplifier 4.5 x 10- 4 Tc Not Determined 1.0 X 10- 3 T 4.0 X 10 -3 T 5.0 X 10- 5 L 2.3 X 10- 4 T 7.0 X 10- 5 L Osc. Conc.f 2 T Osc. Cone.+ 6 T Osc. Cone.+ 6 T L implies longitudinal pumping. e< symbol is used for saturated solutions. Mix up solution at the indicated concentration and allow solute to dissolve as much as possible. dFirst amplifier not used. cT implies transverse pumping. bAll concentrations in units of molarity (moles of solute/liter of solution). aConcentrations given are starting concentrations only. Output wavelength tuning curves can be shifted to longer or shorter wavelengths by increasing or decreasing, respectively, the oscillator dye concentration. Solvent MeOH EtOH EtOH EtOH EtOH EtOH MeOH MeOH EtOH MeOH Dye Solution Concentrations for DL18 Dye Laser.a Dye Stilbene 420 C480 C500 C540A R6G R610 R640 DCM Crystal Violet Oxazine 725 TABLE I. I I ....... \0 ....... -192Frequency mixing is similar to frequency doubling, except that a photon from the Nd:YAG pump laser replaces one of the dye lser photons in the nonlinear process. A dichroic beam combiner is used to overlap the dye fundamental with the Nd:YAG fundamental or second or third harmonic. When mixing with the 1064 nm Nd:YAG fundamental, a half-wave plate rotates the 1064 nm beam polarization to vertical to match the vertically polarized dye laser beam. 2.3 UV400 N2 Laser and DLZOO Dye Laser The two-laser experiments described in Papers II and III, and early photoacoustic spectroscopy studies, used a lower power pulsed dye laser system. The pump laser was a Molectron UV400 N2 laser which produces 10 nsec 400 kilowatt pulses at 337.1 nm. This laser operates with a 30 kilovolt discharge channel containing N2 . The discharge excites N2 molecules and creates a population inversion -3 -3 which lases on the C rru ~ B rrg transition. The N laser was used to pump a Molectron DLZOO dye 2 laser which is shown schematically· in Fig. 3. The DLZOO consists of a single dye cell inside a cavity formed by a grating and output coupling mirror. A beam expanding telescope is used to illuminate more lines on the grating and reduce beam divergence. Using a variety of dyes, this N laser pumped dye laser system produces output in the 2 -193range of 360-900 nm and a maximum pulse energy of 300 ~J. An accessory turret containing 5 KDP crystals can be used to frequency double the dye laser output, producing UV down to 260 nm with a maximum pulse energy of 25 ~J. 3. Crystal Phasematching Angle Control For continuous wavelength scans in the ultraviolet using the DL18 dye laser, both the grating and crystal angle must be scanned simultaneously. The grating scan is con- trolled by the scan circuitry on the prototype board described in the thesis of Ronald Rianda. The crystal scanning system which I developed is described in this section. The grating scan in the DL18 incorporates a sine drive which transforms the rotation of a stepping motor shaft into a linear wavelength scan. The prototype board designed by Ronald Rianda generates periodic pulses which drive the stepping motor for the wavelength scan. Every 0.01 nm, the scan circuitry generates an interrupt which tells the microcomputer to collect data and update the wavelength displayed on the terminal. The machine language program which controls the wavelength scan is called STPRS. The crystal angle is a complicated, nonlinear function of wavelength. · We chose to model this function over small wavelength intervals as a polynomial in grating counter units. (Grating counter units equal the wavelength in angstroms -194 multiplied by the grating order . ) This polynomial is de- termined by a least squares fit of experimentally determined data points of grating counter units and crystal angle. Acceptable wavelength scans can be obtained by taking data points every 25 or SO grating counter units and using a 5th order polynomial. Data collection and curvefitting are performed using the BASIC program CRYST . for data entry: Three options in CRYST are available terminal entry where the user types in both grating counter unit and crystal angle, computer - assisted data entry where the computer scans the grating and the user types in crystal angles, and data modification which allows modification of existing data. Immediately after entry, the data are stored in DEBBI single precision floating point format. Grating counters units are stored starting at BE80 and crystal angles starting at BEOO. The data are shifted by subtracting off the average to increase the accuracy of the fit. The shifted grating counter unit data are scaled by dividing by 256. A polynomial of order 1 to 5 is determined from a least squares fit to the shifted and scaled data . The resulting coefficients are adjusted for the scaling and shifting, converted to SBC310 highspeed math board floating point format by user subroutine USR4 contained in program SCSU4, and stored starting at memory location BF74 . The average of the grating counter unit data XT is stored at BF70. -195 A modified version of STPRS was written, called LSCAN, which incorporates the crystal scan routines . The crystal angle polynomial is evaluated every 0.01 nm, and the result is sent to the DL222 crystal scan control in the DL18 dye laser. The current grating counter unit PCNTR is shifted by subtracting XT and used as the argument for the polynomial. The polynomial is evaluated in floating point format using Horner's method of expanded parentheses and the SBC310 highspeed math board . The floating point result is converted to 16 bit fixed point integer format. The eight least signifi- cant bits are sent to port A of the Intel 8255 parallel port on the prototype board which is memory mapped at 2FE8. 7405 open collector drivers were used to increase the 8255 output to meet the input voltage and current specifications of the Molectron DL222 digital servo open loop control for the crystal drive. A latch signal from port Bl of the 8255 (memory mapped at 2FE9) latches the eight least significant bits into the DL222. The eight most significant bits are sent in a similar manner through port A of the 8255 and latched into the DL222 using a latch signal from port BO. The Molectron DL222 was originally designed to accept two analog voltages which correspond to the eight least significant bits and the eight most significant bits . An analog to digital converter alternately sampled both voltages and sent the digital data to RCA-CD4042B quad clocked latches. -196RCA-CD4063B four-bit magnitude comparators compared the latch data to the current crystal position stored in RCA-CD40193B presetable binary up/down counters, and increased or decreased the crystal angle until the crystal was at the correct angle. The DL222 was modified to accept digital data directly from the microcomputer. The analog to digital converter· chip Ul5 was replaced with a 16-pin component header having the eight data lines from port A of the 8255 parallel port on pins 2-9. The latch lines from just before test points 2 and 7. U26A and U26B were cut These test points were connected to ports BO and Bl of the 8255 parallel port for software control of the latching operation. The crystal scanning system just described permits scanning over entire dye gain curves while maintaining phasematching of the crystal. When slight adjustments are made in the dye laser optics, the output beam direction can change slightly. This change is easily compensated for by using the "CAL" control on the DL222. Using LSCAN, the crystal is positioned at an angle corresponding to the grating counter unit. The DL222 is put in the neAL" mode, the crystal angle is adjusted for maximum power, and then the DL222 is returned to "OPER" mode. In this way, the same coefficients can be used over periods of several weeks, even though repeated tweaking of the dye laser is required every day. -197- Problems arise occasionally at high power levels. Large dips occur in the output vs. wavelength curve. This is believed to occur because of local heating of the crystal, which introduces changes in the index of refraction due to temperature increases. This effect is minimized by taking the initial polynomial data under conditions similar to those under which the experiment will be carried out. Recently, INRAD announced a new autotracking second harmonic generation system, model S-12. This system auto- matically optimizes the crystal angle for maximum conversion efficiency. A dual element photodiode senses the slight shift in output beam direction when the crystal is not phasematched, and supplies a correction signal to the crystal angle servo motor. Systems of this type which function automatically are reported to be simple to use and reliable. Eventually the polynomial system should be upgraded to a fully automatic 4. sea~ system. Electronics for Two-Laser Experiments Papers II and III presented the results of some two- color time-re~olved spectroscopy experiments. These experi- ments required the synchronization of pulses from two independent lasers with nanosecond precision. The electronics used to trigger the two lasers and collect data were described briefly in the two papers. This section will present a more -198 - in - depth description of the electronics needed to perform those experiments. The UV400 N2 laser can be externally triggered with a +5 to +30 volt pulse with a risetime of less than 1 ~sec. The laser fires 500-1000 nsec later with a specified jitter of less than 4 nsec. more complicated. The MY34 Nd:YAG laser triggering is The flashlamps must fire about 150 ~sec before the Q-switch opens, to allow the maximum ~eposition of energy into the ND:YAG rods. The rear panel of the laser has trigger inputs and synchronization outputs for both the flashlamps and Q-switch. The - output pulse temporal jitter is specified at ±1 nsec when triggered by an electrical pulse with a risetime of <15 nsec . A schematic of the trigger and data collection electronics is shown in Fig. 4. The Nd:YAG laser internal 10 Hz clock serves as the source. trigg~r The Nd:YAG flashlamps are fired internally by this clock, and the lamp sync output triggers a Rutherford 87B pulse generator. This pulse gen- erator supplies most of the nominal 150 ~sec delay required before the Q-switch opens. Since most pulse generators have jitter in delay time on the order of 0.1%, the initial 150 ~sec time period cannot be used to actually trigger the Q-switch. Instead, the initial 150 ~sec delay triggers a pair of fast risetime pulse generators which each generate a delay of <200 nsec before triggering the N2 laser and the -199- Nd:YAG Q-switch, respectively . This allows the 150 ~sec delay to fluctuate on the ~sec timescale, but still keeps the timing between the two laser pulses reproducible on the nsec timescale. In Fig. 4, pulse generators B and C are the fast risetime pair of pulse generators. B is a Hewlett Packard 800SB 20 MHz pulse generator with a risetime s 10 nsec. C is an E-H International 139B SO MHz pulse generator with a risetime $ 6 nsec. In practice, the Rutherford pulse generator (A) has an erratic risetime of approximately 1 ~sec so the C pulse generator is triggered from the B pulse generator's trigger out terminal. This insures that B and C are locked together independent of the output waveform of A. Long-term jitter between pulses, as monitored with a Hewlett Packard 5082-4220 PIN photodiode (-18 volt reversebias, T r1se . < 1 nsec), was less than ±2 nsec, with a short-term (10 sec) jitter of ±1 nsec. dye laser were ~5 Pulsewidths of the doubled output of each nsec. In some experiments where wavelengths were scanned (Paper II), it was necessary to correct signals for fluctuations in laser pulse energy. Since we have only one pyro- electric joulemeter, we designed a detection system which subtracts out the fraction of the total signal due to the N2 dye laser system in real time. The remaining signal from the Nd:YAG dye laser system is stored along with intensity data from that laser. To accomplish this, we fired the N2 -200laser at 20 Hz, twice the Nd : YAG laser repetition rate, by operating pulse generator B in the double pulse mode. The photoacoustic or MPI signals were integrated by a differential gated integrator system which added the signal when both lasers fired and subtracted the signal when only the N2 laser fired . Pulse generator D, a Molectron model 122 dual gate generator, produced the integration gates for the integrators. Pulses were sent alternately to first the "add" gate input of the integrator and then to the "subtract" gate input by a homebuilt pulse router . The pulse router consisted of a flip-flop, triggered by pulse generator B, which selected which channel of a multiplexer would be active. The pulses from pulse generator D passed through the multiplexer and were directed to either the "add" or "subtract" integrator gate inputs. The photoacoustic or MPI signals were initially processed by an Ithaco 4302 dual channel 24dB/octave electronic filter. The signal passed through both channels sequentially. The first channel was operated as a low-pass filter with a gain of 1 or 10. The second channel was operated as a high- pass filter, also with a gain of 1 or 10. Further amplifi- cation and filtering was performed with a Molectron model 131 amplifier/shaper . The integrator was a Molectron model 112 differential gated integrator. Nd:YAG laser pulse energies were measured with a Laser Precision RkP-335 pyroelectric energy probe and Rk3230 -201- readout unit. The pulsed analog output of the readout unit was also amplified and filtered by a second amplifier/shaper and integrated by a differential gated integrator. Analog signals from the two gated integrators were digitized and stored on floppy disk by a microcomputer which also controlled the wavelength scan. The stored signal and laser intensity data were processed at a later time and plotted to produce the final publication spectra. -202Figure Captions Figure 1. Optical schematic of the Molectron MY-34 pulsed Nd:YAG laser. The oscillator cavity is defined by mirrors M1 and M2 . SHG and THG are abbreviations for second harmonic generation and third harmonic generation, respectively. Figure 2. Optical schematic of the Molectron DL-18 dye laser. The letters A through P label the following components: A - beam shutter, B - beam splitter, C- total reflector, D - focusing lens, E - magnetically stirred or flowing dye cell, F - prism beam expander, G - diffraction grating, H - output coupling mirror, J - A/2 plate, K - beam expanding telescope, L - post amplifier flowing dye cell, M - right-angle prisim, N - dichroic beam combiner, 0 - doubling or mixing cry~tal, P - UV bandpass filter . Figure 3. Optical schematic of the Molectron DL-200 dye laser. An etalon was not used in the experiments described in this thesis. The polarizer is only used when the dye laser output is being frequency doubled and the grating counter unit is below 28,000. Figure 4. Schematic of the laser triggering electronics and data collection system used in two-color timeresolved spectroscopy experiments. The operation of this system is described in Section 4. ~ I-' (!) ~ M, POCKELS CELL I I II I I I THG I Nd1 YAG ROO )./4 PLATE )./2 PLATE M2 Y Y I • 1064nm • 355nm I >-l A' I SHG 532nm N 0 tN A~IFIE.R II • ('Q 1-'· 'T1 A' )./2 PLATE OPTICAL SCHEMATIC MY-34 PULSED Nd: YAG LASER 'T.1 N ([) >-! c f-1• (JQ G VACUUM BOX PUMP LASER BEAM OSCILLATOR A ~ AMPLFER AMPLIFIER MIXING BEAM ~ OPTICAL SCHEMATIC DL -18 c p 1 ' ~ N 0 f-'· 'T1 VI (]) >i s:: (1q £TALON GRATING L.,.........,. __j :-::::::::::::::::::::::::::::::::::-:-:-:-.· .. . .L. )( DYE CELL OUTPUT MIRROR POLARIZER VISIBLE OUTPUT FOCUSING LENS NITROGEN LASER PUMP BEAM ~ASPHERIC I• OPTICAL SCHEMATIC DL -200 OOUBLING CRYSTAL VISIBLE OR UVOUTPUT I U1 N 0 -206Trig. In PULSE GENERATOR A .,_~1---- From YAG Flashlamp Sync. Output 150,u.sec delay 20Hz . ~' 50nsec delay -; Trig. In To N2 Laser ..........._ _.._-t .; PULSE GENERATOR 8 ....,. 0 Trig. Out Trigger lOHz 200nsec :I delay ... To YAG PULSE GENERATOR C ~ r- Q- Switch Trigger Trig. In Signal In _ 0 ELECTRONIC FILTER/ AMPLIFIER PYROELECTRIC JOULEMETER Trig. In PULSE GENERATOR 0 Output AMPLIFIER/ SHAPER AMPLIFIER/ SHAPER + . Trig. In Pulse In PULSE ROUTER ~' Output #I ~ Output :1=2 Signal In .... Gate Inputs { ~ ~:1------'1'------ I ... r Signal In .....----1:_.. ~}Gate Inputs GATED INTEGRATOR ~ r- ....,.. MULTIPLEXER and A/0 ~ To Computer Figure 4 .... ~ GATED INTEGRATOR - 207- APPENDIX B: The Design of Photoacoustic Cells for Pulsed Laser Excitation . - 208 1. Introduction At the time the experiments described in this thesis were initiated (1977), photoacoustic spectroscopy experiments almost always used CW light sources modulated at audio frequencies. Since that time, a variety of experiments using pulsed laser excitation have been performed, 1 including the experiments described in this thesis. An overview of the important design criteria for optimizing photoacoustic cells for pulsed excitation has not yet appeared in the literature. The photoacoustic cells (PAC's) used in our experiments have evolved through five general design classes as we attempted to improve the sensitivity of our cells. These classes, labeled PACl to PACS, illustrate important aspects of PAC design . A description of each class and its advantages and disadvantages is presented in the following sections. 2. Photoacoustic Cell Design 2. 1 PAC 1 Our first photoacoustic cell, PACl, followed a design used by Reddy and Berry 2 and is similar to the one described by Kreuzer . 3 It consisted of a cylindrical stainless steel chamber 10 . 2 em long with an inner diameter of 3.81 em, which housed a stainless steel cylindrical tube having a 1.27 em outer diameter, 7.0 em length and 8.38 x 10 -2 em wall thickness, perforated by about 350 randomly distributed - 209holes of 0.16 em diameter. This tube was tightly wrapped with an aluminum-coated mylar film 1 . 27 x 10- 3 em thick . A potential of 300 volts was applied across the cylindrical concentric capacitor formed by the aluminum coating and the perforated tube. A Canberra model 2001A charge-sensitive preamplifier integrated the signal produced when the photoacoustic pressure wave displaced the mylar diaphragm. signal consisted of a few successively damped The oscillations, each full oscillation lasting about 10 ~sec. The sensitivity for pulsed excitation of PAC1 was compared to another cell which contained a 1" diameter Bruel and Kjaer microphone model MDC 4144 . PACl was considerably more sensitive to pulsed excitation . The probable reason is that in the cylindrical system we detected a shock wave which struck the entire area of the cylindrical diaphragm simultaneously, rat~er than a smaller flat area of the planar microphone nonsimultaneously . This produced a shorter, larger amplitude pulse which increased the signal-to-noise ratio. The sensitivity of PAC1, however, decreased with time due to stretching of the mylar diaphragm . New diaphragms were difficult to install and did not produce reproducible sensitivities . 2.2 PAC2 The irreproducible sensitivity of the cylindrical - 210microphone forced us to investigate photoacoustic cells containing commercial microphones. Joseph Barrett recom- mended using a Knowles Electronics mode 1 BT17 59 miniature electret microphone, based on his experience with photoacoustic cell design. 4 - 7 This microphone has dimensions of .792 x .559 x .228 em. A "saltshaker"-like port with a diameter of .476 em is located on the top. The microphone has a flat responsivity of 10- 2 volt/Pascal over a frequency range of SO to 10 4 Hz . current of SO ~A An internal FET preamplifier draws a at 1.3 volts DC. A photoacoustic cell, PAC2, which incorporated a BT1759 micropho~e was designed and built. This cell consisted of a small chamber .53 em long with .24 em diameter openings at opposite ends. Quartz windows were mounted over the openings using expoxy resin. A BT1759 microphone was mounted over an opening in the side of this chamber. It was located as close as possible to the central axis defined by the window openings . This maximized the amplitude of the pressure wave produced by absorption of the laser pulse. PAC2 exhibited excellent sensitivity, approximately equal to PACl. All spectra taken with PACZ, however, con- tained large amounts of constant background signal due to absorption or scattering of light at the windows. Window absorption heats the gas near the window, which produces a pressure wave. Window scattering causes light to strike the cell walls, which also heats the nearby gas to produce -211- a pressure wave. In PAC2, the windows were very close to the microphone, causing the window absorption or light scattering pressure waves to arrive at the microphone simultaneously with the pressure wave produced by direct gaseous sample absorption. The constant background was a significant fraction of the total signal so a major modification was required . 2.3 PAC3 PAC3 was designed to minimize window background signals and still retain the sensitivity of PAC2. This was accom- plished by moving the windows back away from the microphone. The window absorption and scattering pressure wave still reached the microphone, but not until after the pressure wave from the sample absorption had already been detected. By proper gating of the detection electronics, only the sample absorption pressure wave was detected. PAC3 consisted of a stainless steel cell with a 3.8 em long by . 56 em diameter sample chamber. Suprasil 1 windows were mounted at Brewster's angle at each end using Viton o-rings . A BT1759 microphone was mounted over a hole in the cell wall approximately equidistant from both windows . A quartz window was mounted on the side opposite the microphone to allow simultaneous fluorescence measurements . The window noise was drastically reduced in PAC3 compared -212to PAC2. A small residual background signal was still present, however. It was eventually determined that the Brewster angle windows were still contributing scattered light signals. Brewster's angle changes slowly as a function of wavelength and the laser beams are not perfectly polarized. These two effects caused a small amount of light to reflect off the exit window and strike the cell walls. The polished stainless steel surface allowed multiple reflections which caused scattered light to eventually strike the walls near the microphone or the microphone itself. This produced a pressure wave which arrived at the microphone simultaneously with the pressure wave from sample absorption. This effect was small, but still able to overshadow extremely small absorption signals. also too small. The sample chamber diameter was The small diameter required decreasing the 8 mm diameter DL18 dye laser beam to avoid striking the cell walls. This increased the nonlinear absorption effects de- scribed in Paper IV. 2.4 PAC4 The PAC4 class of photoacoustic cells had parallel windows oriented perpendicular to the laser beam, and larger diameter chambers. This eliminated the scattering from the Brewster angle windows of PAC3. The larger diameter allowed reduced intensities because tight focusing was no longer -213required to keep the beam from striking the cell walls. A BT1759 microphone was mounted as close as possible to the laser beam to maximize the amplitude of the pressure wave. Most of the experiments described in Papers I and IV used the PAC4 design. PAC4 cells were constructed from brass or stainless steel 3/8 inch Swagelok fittings. A BT1759 microphone was mounted over a hole in the side of a Swagelok tee using epoxy resin. Before mounting, the side of the tee was machined down to allow positioning of the microphone as close as possible to the laser beam. Quartz windows were epoxied onto 3/8 in Swagelok to 1/8 in female pipethread adapters. Short pieces of 3/8 in tubing were used between the tee and pipethread adapters. 15 em. The overall finished lengths were about The PAC4 cells were inexpensive (<$50.00) and could be constructed from standard fittings in a single afternoon. Their major disadvantage was the difficulty in replacing or cleaning windows. Many samples produce photochemical deposits on the cell windows after irradiation with UV light. The window deposits increase the scattered light background. After extended irradiation, the windows must be removed, cleaned and replaced. Most windows were broken in the process of removing them from the epoxy seals. After replacing the windows, a 24-hour period was required before the cell could be used again. epoxy This time period was necessary to allow the to cure and outgas. -214- 2.5 PAC5 The currently used photoacoustic cell, PAC5, is similar to the PAC4 cells except that 0.5 in stainless steel Swagelok fittings are used. Two pairs of apertures are also mounted inside the cell to decrease scattered light. The two aper- tures near the windows have diameters of .64 em. The inner apertures located near the microphone have diameters of . 71 ern. This choice of diameters insures that a laser beam passing through both outer apertures will not scatter off the edges of the two larger inner apertures. These apertures essentially eliminate all scattered light which could produce detectable pressure waves that reach the microphone simultaneously with the pressure waves caused by sample absorption. The PAC5 cell was used in the experiments discussed in Papers II and III. 3. Conclusions In summary, the general criteria which should be con- sidered when designing photoacoustic cells for use with pulsed excitation are: (1) minimize scattered light, (2) locate the windows far enough from the microphone to allow temporal discrimination between sample absorption and window absorption, and (3) mount the microphone as close as possible to the laser beam to increase the amplitude of the detected pressure wave. The PACS cell is currently our state-of-the-art -215- photoacoustic cell. The sensitivity of this cell could possibly be increased by mounting the microphone inside the cell. In the present configuration, the microphone is mounted over a hole in the cell wall . The hole limits the transmission of the pressure wave to the microphone . Placing the microphone inside the cell would eliminate this problem and also allow the microphone to be closer to the laser beam . If additional pulsed photoacoustic spec- troscopy experiments are attempted which require high sensi tivity, this new configuration should be tested. -216References 1. A partial list of pulsed photoacoustic experiments can be found in Paper IV. 2. M. J. Berry and K. V. Reddy, private communication; K. V. Reddy, PhD Thesis, University of Wisconsin-Madison (1977); K. V. Reddy, Faraday Discuss. Chern. Soc . 67, 188 (1979). ~' 3. L. B. Kreuzer, J. App1. Phys. 2934 (1971). 4. J. J. Barrett and M. J. Berry, App1. Phys. Lett. 34, 144 (1979). i' 395 (1979). 5. G. A. West and J. J. Barrett, Opt. Lett. 6. D. R. Siebert, G. A. West and J. J. Barrett, App1. Opt. ~' 7. 53 (1980). G. A. West, D. R. Siebert and J . J . Barrett, J . App1. Phys. ~' 2823 (1980). -217- APPENDIX C: Wavelength Conversion by Stimulated Raman Scattering. -218Wavelength Conversion by Stimulated Raman Scattering The Molectron Nd:YAG laser, dye laser, and accessories can produce tunable coherent output from about 217 nm to 900 nm by using a variety of dyes, and frequency doubling and mixing in KDP and KPB crystals. The high peak powers available fran Nd:YAG pumped dye lasers and excimerlasers has led to the development of a wide variety of nonlinear optical techniques for converting the output of these lasers to shorter and longer wavelengths. Stimulated Raman scat- tering (SRS) has gained widespread interest because of its 1 2 simplicity and broad wavelength range. ' SRS has been observed in solids, liquids, and gases. It is a nonlinear parametric interaction involving the third3 order nonlinear susceptibility x . Several review articles 3 have summarized the current understanding of SRS. - 5 In simple terms, a pump laser pulse at frequency vp promotes a molecule from initial state, i, to a virtual state. If the pump pulse is intense, a population inversion can result between the virtual level and a final level, f. In hydrogen, for example, states i and f may correspond to the ground and first vibrational states, respectively. Once a population inversion is produced, stimulated emission occurs at a frequency vSl = vp - (vf - vi) = vp - vR. 4155 cm-l For hydrogen, vR = The production of stimulated vSl output has a flux density threshold. Once this threshold is achieved, - 219 - the Sl output increases exponentially with distance from the initial spontaneous Raman output . Sl and the pump beam are in .phase and can beat against one another to produce an electronic polarization in the gas at the Raman frequency vR. This induced polarization interacts with the pump beam to produce sidebands at vp ± vR. The v p + v R sideband is called first anti-Stokes output and denoted as ASl . (The vp - vR sideband is called first Stokes output and labeled Sl . ) The process just described is not exactly correct because it implies that there is a time dependence involved in the various processes . Actually, the generation of ASl is a parametric four-wave mixing process involving simultaneously two vp waves and one vSl wave to produce a vASl wave . ASl can interact with ASO (the pump) to produce AS2 at frequen~y vASZ = vp + zvR. The process repeats itself again and again to produce a large number of AS lines. Higher order Stokes lines can also be generated parametrically. The end result is that once threshold is reached, a distribution of new frequencies is produced with frequencies vn = vp + nvR with n equal to all integers between approximately -5 and 8, depending on the dispersion of the gas. All Stokes and anti - Stokes beams come out collinearwith the pump beam and must be separated with a dispersing prism system. As vp is scanned, all Stokes and anti-Stokes lines also scan simultaneously. H2 is widely used as a SRS medium because of its very large Raman cross section, widely spaced vibrational levels, -220resistance to dielectric breakdown, and low optical dispersion. The low dispersion allows a broad range of Stokes and anti-Stokes lines to be produced. have higher outputs. The low order lines Each successive higher order anti- Stokes line, for example, can be reduced in intensity by 70% of the previous line. 6 Wavelengths as short as 175 nrn have been observed, which correspond to the 5AS line of the RGG second harmonic output. 6 The BAS line of the RGG fundamental has been observed at 194 nrn. In practice, a trade-off is available between conversion efficiency and threshold intensity. Weaker focusing of the pump beam raises the threshold and increases the conversion efficiency. With low power lasers, tight focusing is required to reach threshold. This decreases the conversion efficiency. The H2 pressure is adjusted to maximize the conversion to the selected Stokes or anti-Stokes line. occurs between 100 ~nd 300 psi. Optimum conversion The Stokes lines generally require the higher pressures. A SRS cell was constructed in our laboratory to extend the wavelength range accessible to our laser system. Briefly, it consists of three sections of 1.25 in OD x .120 in wall stainless steel tubing in lengths of 15, 30, and 60 ern. Standard Varian 2.25 in high vacuum Conflat flanges are used to join the sections together. A double-sided flange containing a pressure gauge, fill port, and 2000 psi safety relief valve can be inserted between any two sections. 1 in -221diameter by .5 in thick Suprasil 1 windows are mounted at each end using two additional flanges. The windows are inserted into the flanges from the inside, allowing the windows to seat against a .125 in wide rim, for maximum pressure handling capability. Viton o-rings are located in this rim to provide a vacuum seal. The overall length of the cell is variable, depending on which sections are joined together. The highest outputs were obtained by using the maximum cell length of 105 em and a 50 em focal length lens for.the input pump beam. Figure 1 shows a typical optical arrangement for SRS. The output of the dye laser is focused into the 105 em cell with a 50 em focal length lens. output beam. A similar lens recollimates the A series of four Brewster angle prisms is used for wavelength separation. The first two prisms cause a parallel displacement which is a function of wavelength. An aperture selects the appropriate Stokes or anti-Stokes line. Two additional prisms correct for the change in dis- placement caused by scanning wavelength. The unwanted beams can be very intense so care is required to avoid stray reflections. Figure 2 shows the output pulse energies as a function of input pump pulse energy for low order Stokes and antiStokes lines. A threshold of ~z mJ/pulse is observed using the RGG dye output, H2 pressure of 225 psi, and the optical arrangement just described. After threshold, the lAS and -222- 2AS line intensities increase linearly with pump pulse energy. Other lines, however, are not linear. For some lines, the output actually decreases with increasing pump energy in certain regions. function of H2 pressure. These curves change as a By choosing a pump pulse energy where the derivative of the selected line is 0, the SRS process may actually decrease the effect of pulse to pulse energy fluctuations in the pump pulse. In the wavelength region accessible to the Nd:YAG/dye laser system, 217 to 900 nm, the SRS outputs are lower than the corresponding outputs using various dyes and mixing or doubling. If high pulse energies are not necessary, however, the SRS process is extremely simple to use. One dye like RGG can produce a wide range of wavelengths. Scanning wave- lengths is simple, since maintaining the phasematching angle of a nonlinear crystal is not required. For wavelength regions outside the· 217 to 900 nm region, SRS is definitely attractive. We were able to easily obtain ~5~J of output at 192 nm using the 4AS line of doubled RGG. 2.5 mJ at 1061 nm was obtained using the 25 line of the RGG fundamental. Production of shorter wavelengths is possible by changing window materials and using an evacuated beam path after the cell. The increased wavelength coverage and simplicity of use make the SRS technique an attractive accessory for our laser system. Additional information about SRS technique and theory can be found in References 7-15. -223References 1. J. Paisner and S. Hargrove, Energy Technology Review (published by Lawrence Livermore Laboratory), March 1979, p. 1. 2. R. L. Byer, Electro-Optical Systems Design, Feb. 1980, p. 24. 3. N. Bloembergen, Am. J. Phys. 35, 989 (1967). 4. W. Kaiser and M. Maier, Laser Handbook, Vol. II, edit. by F. T. Arecchi and E. 0. Schulz-Dubois (North-Holland, Amsterdam, 1972). 5. R. L. Byer and R. L. Herbst, in "Topics in Applied Physics" series, Vol. 16, Nonlinear Infrared Generation, edit. by Y. R. Shen (Springer-Verlag, Berlin, 1977) pp. 81-137. 6. V. Wilke and W. Schmidt, Appl. Phys. 7. P. Rabinowitz. A. Stein, R. Brickman and A. Kaldov, ~' 177 (1979). Opt • Lett • l, 14 7 ( 19 7 8 ) . 8. T. R. Loree, R. C. Sze and D. L. Barker, Appl. Phys. Lett. l!, 37 (1977). 9. V. Wilke and W. Schmidt, Appl. Phys. 16, 151 (1978). 10. T. R. Loree, R. C. Sze, D. L. Barkev and P. B. Scott, IEEE J. Quant. Elect. QE-15, 337 (1979). 11. M. Bierry, R. Frey and F. Pradere, Rev. Sci. Instrum. 48' 733 (1977). 12. R. W. Hellwarth, Phys. Rev. 130, 1850 (1963). 13. Y. R. Shen and N. Bloembergen, Phys. Rev. 137, A1787 (1965). -22414 . P. Rabinowitz, A. Ka1dor, R. Brickman and W. Schmidt, Appl. Opt. 15 . ~' 2005 (1976). W. Hartig and W. Schmidt, Appl. Phys. 18, 235 (1979). -225Figure Captions Figure 1 . Typical optical arrangement for wavelength conversion by stimulated Raman scattering. The H2 Raman shifter is filled with between 100 and 300 psi H2 . Pl. Right-angle turning prisms are labeled by Two SO em focal length lenses labeled L focus the dye laser output into the Raman shifter and · recollimate the output beams. Four Suprasil 1 Brewster angle prisms labeled P2 are used to separate out the desired Raman-shifted line. Figure 2. Output pulse energies as a function of input pump pulse energy for low order Stokes and anti-Stokes lines produced by stimulated Raman scattering in H2 . The S64 nm pump pulse was supplied by the DL18 dye laser using R6G and focused .using a SO em focal length lens. The H2 pressure was 22S psi. (A) Stokes lines and pump output. The symbols designate the following output lines: 0 - 1st Stokes, o- 2nd Stokes, 1:. - 3rd Stokes, •- pump output. (B) Anti- Stokes lines. The symbols designate the following output lines: 0 - 1st anti-Stokes, Stokes, 6 - 3rd anti -Stokes, o- 2nd anti- • - 4th anti -Stokes. The pulse energies of the higher order Stokes and anti-Stokes lines were increased by the indicated multiplicative factor before plotting . 1-'· 'Tl I-' H2 Raman Shifter Aperture I (J\ N I N PI (1) Prism Wavelength Separator Dye Laser 1-i ~ cro Nd=YAG Laser -2272.0r---------------------------------------~ Pump Output (x0.5) \ A 1.5 1.0 -:> --3S(x5) E ......... >.. e' 0 .5 Q) c: w Q.) (/) ::3 a.. - - 0~~~_.------~----~~----~------~----~ ::l a. 8 ::3 0 1.0 0.5 10 20 30 40 Input Pulse Energy /mJ Figure 2 50 60 -228- APPENDIX D: Detection of the 3A State of CS 2 by Multiphoton Ionizatton. -22 93 of the A State of cs 2 by Multiphoton Ionization Ronald Rianda,* David J. Moll and Aron Kuppermann Arthur Amos Noyes Laboratory of Chemical Physics** California Institute of Technology, Pasadena, CA 91125, USA Received The application of the technique of resonance-enhanced multiphoton ionization (REMPI) spectroscopy to the detection of triplet states via a first photon resonance is examined. It is shown to work well for the a3A2 state of cs 2 . This technique promises to permit the detection of spin-forbidden transitions at high resolution and with great sensitivity . Work performed in partial fulfillment of the requirements for a PhD in Chemistry at the California Institute of Technology. ** Contribution No. 6082. -230. 1-10· h as b een utilized extensively The REMPI tee h n1que for the detection of two-photon excitations. However, it has not been employed for the investigation of weak onephoton absorptions, such as singlet-triplet transitions. 10 Such an application could lead to a useful method of studying these kinds of processes, and it is examined in the present paper. The near-ultraviolet absorption bands of cs 2 extending from 390 to 330 nm have been the subject of extensive ex. t a 1 an d t h eoret1ca . 1 1nvest1gat1on . . . per1men . 11-18 The vibra- tional and rotational structure of the bands indicates that the spectrum results from transitions from a linear ground 1 + 13 state ( rg) to a bent excited state of B2 symmetry. Douglas and Milton 14 and Hougen 15 have studied the pronounced Zeeman effect of the bands and have shown that it can only be understood if the observed B2 state is a component of a 3A state with substantial triplet splitting. Differential 2 cross section measurements performed by Flicker et a1. 18 using low - energy, variable-angle electron-impact spectroscopy are also consistent with assignment of the transition as singlet-triplet in nature. REMPI spectroscopy has proven to be a very sensitive . f or stu d y1ng . . . . 1-10 tee h n1que two-p h oton e 1 ectron1c trans1t1ons. Parker et a1. 19 have shown that when the transitions from the two-photon excited state to the continuum are saturated, -231 - which is generally the case at the intensities normally used for such experiments, the weak two-photon process is rate-determining, and the ion current is proportional to the rate of the two-photon resonance excitation. In the present case, the initial one -photon singlet-triplet transition is strongly forbidden (f < 5 x 10 -7 in the spectral region studied), 16 and subsequent excitations within the triplet manifold are expected to be many orders of magnitude faster. Therefore, the initial step is the rate-limiting process, and the spectrum of the singlet-triplet transition is generated as the laser wavelength is scanned. The experimental apparatus used differs somewhat from that reported by other researchers. The laser system con- sisted of a Molectron DL 200 dye laser pumped by a UV400 nitrogen laser. Output power in the spectral region studied, 375 to 360 nm, varied monotonically from 30 kW to 5 kW using the dye PBD at 20Hz. The laser beam was focused with a quartz lens (f = 50 mm) into a gas cell containing two parallel plates 0.75 in. square separated by 0.75 in. The charge pulse is detected by a Canberra 2001A charge-sensitive preamplifier with a nominal sensitivity of 10 V/picocoulomb. The preamplifier output and the output from the pyroelectric joulemeter, which monitored the laser-pulse energy, were directed into a dual-channel, gated differential integrator system. A microcomputer system digitized and stored the data and controlled the laser wavelength scan. pressure was 5 torr. The sample -232A typical REMPI spectrum of cs 2 is shown in Fig . 1. The spectrum displays the sharp structure characteristic of transitions to the 3A2 state superimposed on a broad, intense doublet. The peaks assigned by Kleman 13 and Douglas and Milton 14 using absorption techniques appear in our spectra with positions agreeing within our wavenumber calibration accuracy of 4 em -1 . Representative results for the r and IT bands are summarized in Table I. The band oscil- lator strengths measured by Barrow and Dixon 16 are also listed to give an indication of the sensitivity of the REMPI technique. We detect vibronic transitions having oscillator strengths as low as 2.7 x 10- 8 • An intense doublet has been observed at 55,002 and 55, · 197 em -l b y P r1ce an d s·1mpson. 20 Bot h groups ass1gne · d t h.1s transition as a ~g orbital to sulfur 4s Rydberg excitation. This transition is .forbidden ( 1 r+g + 1rr g ) for single photon excitation but can acquire intensity through vibronic coupling with either the v 2 c~u) or v 3 (cr:) vibrational modes. The transition is allowed for two-photon excitations, however. It is likely that the two-photon singlet+singlet transition accounts for the doublet observed in our spectra at about 367.2 and 365.5 nm, corresponding to two-photon transition energies of 54,466 and 54,720 cm- 1 , respectively. The accuracy of these values is affected by the presence of the superimposed one-photon bands. The differences between the average of our doublets and those obtained by Price and -233Simpson 20 and Rabalais et a1 . 21 are 506 and 370 cm- 1 , respectively. The latter correlates well with the v 2 vibrational frequency of 397 cm- 1 for the ground electronic state. Several peaks are observed between 368 and 369 nm that, because of their positions and intensities, cannot be -1 + 3 assigned to the X rg ~ a A2 excitation . This structure may -1 -1 + also result from the two-photon X rg ~ B g transition. The peaks are separated by approximately 36 cm- 1 , suggesting that they are due to rotational structure arising from a slightly bent configuration in the excited state. Such an explanation has been offered by Price and Simpson 20 for the 40 cm-l spacing between band heads in the 208 nm band. In summary, we have shown that REMPI spectroscopy is a very sensitive technique for studying spin-forbidden singlephoton transitions. The current availability of far more intense pulsed dye lasers promises applicability of this technique to other molecular systems for which the spinorbit coupling is much weaker than for cs 2 . At sufficiently high intensities, however, the nonresonant MPI process will dominate the signal and limit this technique. 22 Below such intensities and under saturation conditions, the different power dependencies of the initially spin-forbidden process vis-a-vis that of any energetically coincident spin-allowed two-photon processes should allow deconvolution of the spectra, yielding both the single-photon spin-forbidden - 234 spectrum as well as the two-photon spin-allowed spectrum. Our measurements indicate that in our experimental arrangement over the pulse energy range 3 to 24 ~J/pulse the 06-ITl spin-forbidden transition signal varies as laser intensity to the 3/2 power . Such dependence, under saturation con- ditions, for a first photon resonance multiphoton process . 23 ' 24 and attributed to features has been observed prev1ously of the focusing geometry. 23-25 The two-photon spin-allowed transition signal, measured at 366.9 nm where the spinforbidden transition contribution is assumed to be small (since there is only little sharp structure characteristic of the a3A2 state at this wavelength), varies approximately as the laser intensity squared . Furthermore, comparison of the resulting intensities of the one-photon spin-forbidden transitions with those of a pulsed ultraviolet laser photoacoustic spectroscopy technique developed recently in our laboratory may furnish information about the dynamics of energy relaxation from these electronically excited states. Acknowledgments ~ This research was supported in part by the U. S. Department of Energy Grant No . EX - 76-G - 03-1305. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily refleet the view of DOE. Partial support was also provided by -235the U. S. Department of Energy Contract No. DE-AS0376SF00767, Project Agreement No. DE-AT03-76ER72004. Code: CALT-767P4-181. Report -2361 TABLE I. Transition wave numbers (in cm· ) and oscillator strengths for the X ~; 1 - i '~ ~ and n bands of CSz. Present Work IOemana Douglasb ' 10 f-rv" Barr owe ~Bands 05-02 26,898 26,894.9 26,901.25 0.99 06-02 27,188 27,183.7 27,190.06 2.88 07-02 27,469 27,467.1 27,471.96 4.54 03-00 27, 101. 8 0.002 04-00 27,402.5 o. 007 05-00 27,689 27,698.0 27,703.29 0.029 13-02 26,986 26,981.8 26,990.39 0.27 14-02 27,281 27,278.6 27,285.22 1.01 15-02 27,567 27,564. 9 06-03 26,790 26788.0 07-03 27,074 27, 068. 9 08-03 27,346 27,342.9 04-01 27,014 27,013. 8 05-01 27,313 27,309. 9 06-01 27,601 27,599.6 14-03 26,884 26,882.1 15.03 27,168 27,166. 5 16-03 27,432 27,430.1 17-03 27,676 27,675.0 1. 98 n Bands 26,886.66 27,397.3 13-01 14-01 27,314.46 27,694 a Reference [13]. b Reference [14]. c Reference [16]. 27,693.4 27,697.59 -237References 'V\JVI..IVVVVVI 1. M. Lambropoulos, S. E. Moody, W. C. Lineberger, and S. J. Smith, Bull. Am. Phys. Soc. 18 (1973) 1514. 2. M. Lambropoulos, S. E. Moody, S. J. Smith, and W. C. Lineberger, Phys. Rev. Lett. 35 (1075) 159. 3. G. Petty, C. Tai, and F. W. Dalby, Phys. Rev. Lett. 34 (1975) 1207. 4. P. M. Johnson, M. R. Berman, ·and D. Zakheim, J. Chern. Phys. 62 (1975) 2500. 5. P. M. Johnson, J. Chern. Phys. 62 (1975) 4562. 6. P. M. Johnson, J. Chern. Phys. 64 (1976) 4143. 7. P. M. Johnson, J. Chern. Phys. 64 (1976) 4638. 8. D. H. Parker, s. J. Sheng, and M. A. El-Sayed, J. Chern. Phys. 65 (1976) 5534. 9. D. H. Parker and P. Avouris, Chern. Phys. Lett. 53 (1978) 515. 10. R. E. Turner, V. Vaida, C. A. Molini, J. 0. Berg, and D. H. Parker, Chern. Phys. 28 (1978) 47. In this work, a triplet state of pyrazine was detected during the course of a REMPI search for the analog of the Eg state observed in benzene by Johnson 7 via a two~photon resonance. 11. L. N. Liebermann, Phys. Rev. 60 (1941) 496. 12. A. E. Douglas, Can. J. Phys. 36 (1958) 147. 13. B. Kleman, Can. J. Phys. 41 (1063) 2034. -23814. A. E. Douglas and E. R. V. Milton, J. Chern. Phys. 41 (1964) 357. 15. J. T. Hougen, J. Chern. Phys. 41 (1964) 363. 16. T. Barrow and R. N. Dixon, Mol. Phys. 25 (1973) 137. 17. M.-J. Hubin-Franskin and J. E. Collin, J. Electron Spectrosc. 7 (1975) 139. 18. W. M. Flicker, 0. A. Mosher, and A. Kuppermann, J. Chern. Phys. 69 (1978) 3910. 19. D. H. Parker, J. 0. Berg, and M. A. El-Sayed, in: Advances in Laser Chemnstry, ed., A. H. Zewai (SpringerVerlag, New York, 1978), pp. 320-335. 20. W. C. Price and D. M. Simpson, Proc. R. Soc. London, Ser. A: 21. 165 (1938) 272. J. W. Rabalais, J. M. McDonald, V. Scheer, and S. P. McGlynn, Chern. Rev. 71 (1971) 73. 22. L. Zandee and R. B. Bernstein, J. Chern. Phys. 71 (1979) 1359. 23. G. Hancock, J. D. Campbell and K. H. Welge, Opt. Commun. 16 (1976) 177. 24. T. P. Cotter and W. Fuss, Opt. Commun. 18 (1976) 31. 25. S. Speiser and J. Jortner, Chern. Phys. Lett 44 (1976) 399. -239Figure Caption 'V\J\IV\IV\/\/\ Figure 1. Mu1tiphoton ionization spectrum of the single1 ~ -a 3A t rans1t1on · · · d photon X- ~+g ~ ~ o f CS 2 super1mpose 2 -1 + 1 on the two-photon X rg -+ B ITg transition. The ordinate is corrected for the variation of laser pulse energy with wavelength. was 5 Torr. The sample pressure The band assignments at the top of -1 · + 3 the figure are for the X rg -+ a A2 transition from Kleman 13 where r, IT, ~, ... , correspond to k' = t" = 0,1,2,3,. 00' and the symbol 12-r3 implies v'1 = 1, v'2 = 2, v"2 = 3' and v3 = v"3 = v"1 = 0. k is the symmetric top quantum number and t is the quantum number describing the vibrational angular momentum. correspo~d Vibrational modes 1, 2, and 3 to the symmetric stretch, bend, and antisymmetric stretch, respectively, with double prime and single prime corresponding to ground and excited electronic states, respectively. ~ 1 8.lnl3~d INTENSITY (ARB. UNITS) 0 0 U'1 06-cp3 05-I2 14-7T3 ~~ ~ ~ fTl~ r<.e t 05-~2 08-~4 ~~- fTl z G) -f :r: ~L z - _____.;:r=- 15-¢3 -.,j J ~ 14-~ !..l~ :U::I::::t I08-7T3 I Q') U'1 (JJ ~ 06-1rl (JJ 05-IO I =r Q') II I =:t -otz- -241- Proposition I: Abstract: A Direct Measurement of the Internal Conversion Rate in the s 3 State of Naphthalene Using Picosecond Two-Color Time-Resolved Multiphoton Ionization Spectroscopy . It is proposed that the picosecond two-color time- resolyed multiphoton ionization technique be used to investigate internal conversion processes in naphthalene. A direct measure - ment of the s 3 internal conversion rate in naphthalene - h 8 and -d by this method will provide important information about the 8 dynamics of radiationless processes in this molecule. -242Radiationless processes originating in singlet states above the first excited singlet in large molecules are 1 generally very fast . Internal conversion rates for transfer of energy from singlet states Sn for n > 1 are typically 1o 10 -1o 13 sec-l The experimental investigation of the dynamics of these processes is difficult because their rates are significantly faster than the rates of fluorescence (<10 9 sec- 1 ). This discrepancy in rates forms the basis for "Kasha's Rule" which states that polyatomic molecules emit only from the lowest electronically excited level of a given multiplicity . 2 Violations of this rule have been observed only in a few selected molecules such as thiophosgene, 3 pyrene, 4 naphthalene 5 and isoquinoline 6 in which emission was observed from S2. In this proposition, it is proposed to use picosecond time-resolved two-color multiphoton ionization (MPI) s~ectroscopy for studying internal conversion processes 12 on the losecond timescale. This technique has recently been used to measure isomerization rates in stilbene. 7 The results of Paper III of this thesis demonstrated that the nanoseconrl version of this technique could measure intersystem crossing rates on the 10 -9 second timescale in aniline. the vibrationless In that study, a PUMP pulse populated level of s 1 . pulse, ionized molecules in s 1 • occurred at a later time, occurred. ~t A second pulse, the TRANSFER When the TRANSFER pulse > 20 nsec, no ioni zation For values of ~t > 20 nsec, the initially excited -243- singlet state has crossed over into a triplet state with 7200 cm-l of vibrational energy. The explanation for the lack of ionization was that the Franck Condon factors for direct ionization from the vibrationally excited triplet raised the vertical ionization potential for that state. This prevented direct ionization. In addition, a fast dissociation process apparently competed with vibrational autoionization, which prevented the occurrence of that ionization mechanism. Similar effects were also observed in benzene. In both benzene and aniline, the ionization signal decayed exponentially as a function of ~t. The decay time constants for both molecules agreed with previously observed fluorescence decay time constants for those molecules. The change in Franck Condon factors for ionization, which occurs during.intersystem crossing should also occur for internal conversion processes. Therefore, the ionization signal is expected to change during internal conversion just as it does for intersystem crossing. This change should be larger when the TRANSFER pulse excites the molecule to a level just above the adiabatic ionization potential. Under those circumstances, the change in Franck Condon factors may raise the vertical ionization potential above the level ex cited by the TRANSFER pulse, and therefore decrease the probability of direct ionization. -244The time-resolved MPI technique has several advantages over commonly used emission techniques. (1) It should be capable of resolving processes with rates up to 10 13 sec-! when sub-picosecond laser techniques are used. Fast photo- multipliers are limited by electron space charge effects to timescales >lo- 11 sec. Streak cameras can resolve faster events but their sensitivity is lower . sensitive. (2) It is extremely Single electrons are detected easily and electric fields can be used to extract 100% of the electrons from the ionization region. Low solid angle collection efficiencies inherent in fluorescence detection techniques are not present in MPI techniques . (3) The time-resolved MPI technique should be applicable to a large number of molecules. The photoioni - zation process is not limited by symmetry or selection rules. All partial ionization cross sections to a given ionic state are of the same order of magnitude. 10 Naphthalene is an ideal molecule to study using picesecond two-color time-resolved MPI spectroscopy. It has a low IP (8 . 15 eV), 8 states s 1 , s 2 and s 3 are accessible to current picosecond lasers, 8 and studies of radiationless transitions in that molecule have been reviewed recently. 11 The dominant nonradiative charinel in naphthalene at low excess energy above the s 1 origin (32,020 em -1 ) is s 1 ~ T1 intersystem crossing.!! Stockburger and co-workers have presented evidence for additional intersystem crossing -245processes when the excited vibronic level of s 1 is in resonance with T2 or T 3 . 12 Lim and co-workers observed a change in slope of kNR vs. excess energy at the threshold 13 of the s 2 state (35,910 cm- 1 ). ·They explained this effect by pointing out that direct s 0 + s 1 optical absorption produces a different vibrational distribution than s 2 + s 1 internal conversion. Wannier and co-workers 6 observed weak resonance fluorescence from s 2 . This emission had an anomalously long lifetime of about SO nsec. At excess energies of 10,000 em -1 or higher, a rapid decrease of kNR with excess energy was observed by Lim and co-workers. 13 This was explained as being due to the onset of a fast s 1 + s 0 internal conversion. 14 For excitation in the region of s 3 , the lifetime of fluorescence observed from s 1 decreased rapidly with excess energy. 9 The experimental results indicate that naphthalene has a fast nonradiative process which begins only when it is excited into the region of s 3 . The time-resolved MPI technique may be able to measure the actual rate of this process which is probably an internal conversion process. The proposed MPI experiment is conceptually simple. A picosecond laser pulse at 456 nm (photon energy slightly larger than one-third of the ionization potential) is frequency doubled in KPB and focused into a parallel plate MPI cell. The unconverted 456 nm fundamental would be split off before the cell, pass through an adjustable delay line, and be recombined with the doubled beam before entering the MPI -246The cell would be heated to ~35°C to increase the naphthalene sample pressure to ~0.2 torr. 5 A chargecell. sensitive preamplifier would integrate the charge produced by ionization. The frequency doubled pulse would excite 5 3 • fundamental pulse would ionize molecules from 5 3 • The If the fundamental pulse is delayed with respect to the doubled pulse, allowing internal conversion to occur, the ionization probability should decrease due to the Franck Condon factor argument given earlier. The lifetime of the 5 3 state would be determined by measuring the ionization current as a function of the delay between the frequency doubled pulse and the fundamental pulse. The 400 to 500 nrn wavelength range is not easily accessible by typical picosecond pulse generation techniques. Picosecond pulses a~ 420 nm have been produced by synchronously pumping a dimethyl POPOP dye laser with a frequency doubled mode-locked ruby laser. 15 The shorter output wavelength of this laser system may decrease the expected change in ionization after internal conversion because molecules would be excited 5700 cm-l above the IP. The travelling-wave parametric process demonstrated 16 . . an d Ka1ser prov1. des a b etter a lt erna t.1ve. b y 5 e1. 1me1er Their technique converts the frequency doubled output of a mode-locked Nd: glass laser to picosecond pulses which are tunable from 2700 to 17,000 cm-l The wavelength is selected -247by angle tuning the two LiNb0 3 crystals. The 456 nm funda- mental pulse would be obtained by adjusting the crystal angles to produce 10,965 cm-l output and frequency doubling in KDP. These pulses would be amplified in a commercially available Coumarin 450 dye amplifier system 17 pumped by the third harmonic of a Nd:YAG system. The final pulse energy can be conservatively estimated in the following way. Start with a 10 mJ pulse from a typical 18 mode-locked Nd:glass laser with a 6 picosecond pulsewidth. Conversion efficiencies for each conversion porcess are estimated as: frequency doubling - 10%, parametric process - l%, frequency doubling- l%, 15 amplify - 10 6 %. 16 , 18 Using these estimates, the final pulse energy at 456 nm is ~1 mJ. Fre- quency doubling this pulse with 1% efficiency gives a 10 ~J UV pulse for exciting the s 3 state in naphthalene. The 1 mJ 456 nm pulse would be used to ionize molecules in s 3 . The two-color time-resolved MPI experiment on aniline described in Paper III of this thesis used only 0.4 and 13 ~J pulses to excite s 1 and ionize, respectively. Naphthalene is known to ionize with very high efficiency 20 using two-photon MPI processes which are resonant with s 1 • The s 0 + s 3 transition has a larger absorption coefficient . 1·1es t h at t h e S resonant MPI t h an S 0 + S1 , 12 wh.1c h 1mp 3 process should be even more efficient. Therefore, it seems resonable to assume that the proposed experimental method will provide more than adequate sensitivity. 16 -248- After an initial measurement of the s 3 internal conversion rate in naphthalene, it would also be interesting to investigate the same process in naphthalene-d 8 . Inverse isotope effects for the nonradiative rates (k~R/k~R < 1) have been observed in fluorescence studies for excess energies ~ . . ll 5 00 em -l a bave t h e S 1 or1g1n. Th e s1ze . o f t h.1s e ff ect could be measured directly in s 3 internal conversion processes using the MPI technique. - 249References 1. s. K. Lower and M. A. E1-Sayed, Chern. Rev . ~, 199 (1965). ~' 2. M. Kasha, Disc. Faraday Soc . 3. S. Z. Levine, A. R. Knight and R. ·p. Steer, Chern. Phys. Lett. ~, 73 (1974); D. Phillips and R. P. Steer, J .. Chern. Phys. 4. 14 (1950) . £I, 4780 (1977). P. A. Geldof, R. P. H. Rettschnick and G. J. Hoytink, Chern. Phys . Lett. i, 59 (1969); H. Baba, A. Nakajima, M. Aoi and K. Chihara, J. Chern. Phys. 55, 2433 (1971); T. Deinum, C. J. Werkhoven, J. Langelaar, R. P. H. Rettschnick and J. D. W. VanVoorst, Chern. Phys. Lett. ~~ 5. 189 (1971). P. Wannier, P. M. Rentzepis and J. Jortner, Chern. Phys. Lett. 6. ~, 193 (1971). P. M. Felker and A. H. Zewail, t'Jet Spectroscopy of Isoquinoline", accepted for publication in Chern. Phys. 7. B. Lett. (a) A. H. Zewail, results of this study will be presented at The Royal Soc. of Chern., Faraday Division, Disc. No. 75 (Intramolecular Kinetics), University of Warwick, England, 18-20 April, 1983. (b) B. I. Greene and R. C. Farrow, results summarized in Laser Focus 19, 24 (1983). D. W. Turner, C. Baker, A. D. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy (Wiley, London, 1970), p. 321. 9. U. Laor and P. K. Ludwig, J . Chern. Phys. 54, 1054 (1971). 10. J. H. D. Eland, Photoelectron Spectroscopy (Wiley, New York, 1974), p. 58. 11. P. Avouris, W. M. Gelbart.and M.A. El-Sayed, Chern. Rev. zz, 793 (1977). -25011. M. Stockburger, H. Gattermann and W. K1usmann, J. Chern. Phys. 12. ~' 4519 (1975); J. Chern. Phys. ~' 4530 (1975). J. C. Hsieh, C.-S. Huang and E. C. Lim, J. Chern. Phys . .§_Q_, 4 3 4 5 ( 19 74) • 13. J. C. Hsieh and E. C. Lim, J. Chern. Phys. 61, 736 (1974). 14. M. R. Topp and P. M. Rentzepis, Phys. Rev. 3A, 358 (1971). 15. A. Sei1meier and W. Kaiser, App1. Phys. 23, 113 (1980). 16. Available from Quanta-Ray, Inc. See Laser Focus, Dec. 1982, p. 62. 17. Quantel International markets a mode-locked phosphate glass laser with 6-10 ps. pu1sewidth and 5-50 mJ output/ pulse. 18. E. P. Ippen, J. M. Weisenfeld and D. J. Eilenberger, "Sub-Picosecond Optics, Techniques and Applications", presented at Int. Conf. Lasers '79, Orlando, Florida, Dec. 17-21, 1979. 19. Con£. Proc., p. 37. D. E. Cooper, R. P. Frueholz, C. M. Klimcak and J. E. Wessel, J. Phys. Chern. 86, 4892 (1982). -251- Proposition II: Abstract: Collisional Relaxation of Rovibrational Levels of Formaldehyde Prepared by Stimulated Emission Pumping. It is proposed to investigate the collision- induced R-R rotational energy redustribution processes in excited vibrational levels of formaldehyde. Specific ro- vibrational levels will be prepared by stimulated emission pumping. Laser-induced fluorescence will be used to monitor the decay of the initially populated level and the appearance of population in other rotational states. - 252The inelastic cross section for rotational transitions during energy transfer collisions are of fundamental importance . . f or gas k 1net1cs .l Laser 1sotope . . separat1on, gas lasers an d interstellar modeling are examples of areas which need accurate cross sections for these processes . Microwave or mirro- wave-infrared double resonance and molecular beam techniques have been used to study rotationally inelastic scattering in noble gas-NH 3 collisions; 2 CsF collisions with He, CH , N , 4 2 3 . 4 N20, CF 3Br, etc.; He collisions with N , CO and CH , and 2 4 5 NO collisions with Ar. Many other experimental studies could be listed. However, there remains a .large class of molecules which are inaccessible to most experimental techniques. Sophisticated theoretical methods have been applied to molecules in this class. 6 - 9 Formaldehyde, in particular, has been the subject of several theoretical studies because of its presence in interstellar space and the lack of experimental data on its rotational inelastic cross sections. 7 In this proposition it is proposed to study rotational relaxation processes in formaldehyde. A stimulated emission 10-14 would prepare formaldehyde molecules p~ping (SEP) process in a specific rovibrational state in the ground electronic state. A PROBE pulse would detect the rotational population a short time later (20-500 nsec) through a laser-induced fluorescence (LIF) technique. 11 ' 12 By varying the PROBE wavelength and its delay after the SEP process, the time -253- dependences of the populations in specific rotational levels could be monitored. The rate of disappearance of the initial level prepared by SEP, the the appearance of population in other levels, would be compared to the results of theoretical studies. 6 ' 7 The SEP-LIF technique has several advantages which should be pointed out. (1) It is highly selective for populating specific rovibrational levels. (2) A large number of ground electronic state levels are accessible with the SEP process. (3) It allows the use of bulk gas samples. The number of collisions before detection can be controlled by changing the delay between the SEP process and the PROBE pulse. This allows "single-collision" processes to be monitored in bulk gas (4) The temperature can be varied easily. samples. The A.1 A2 and X1A1 states of formaldehyde have been . . 13,16,17 spec. 15 an d emJ.ssJ.on studied in detail by a b sorptJ.on troscopy. The spectrum of formaldehyde is that of a slightly asymmetric (near-prolate) top, with rotational energy levels labeled by a total angular momentum, J, and its projector, Ka, on the CO symmetry axis such that J ~ Ka ~ 0. For Ka > 0, each J,Ka level is split by the small asymmetry into evenand odd-parity states. This splitting is labeled by Kc which is a valid quantum number only for the oblate symmetric top limit. is JK The conventional notation for these rotational levels K . a' c -254- The fluorescence bands originating in the A1A1 state exhibit perpendicularly polarized transitions with selection rules ~K = ±1 and ~J = 0,±1. 17 Transitions are labeled by . ~K Ni a . a~JK" K"(J") where N{ indicates an electronic transition a' c involving vibrational mode N with j and i quanta in the upper and lower states, respectively, and the double primes designate lowe r state labels. The proposed experiment can be broken down into two processes: the preparation of the initial state by SEP, and the detection of molecules in specific rotational levels. The SEP process used to prepare the initial state would be similar to that of Reisner and co-workers. 13 The PUMP pulse, which ... 1 populates a level of the A A state, would be supplied by a 2 Nd:YAG laser pumped dye laser. The dye laser would be equipped with an intracavity etalon (output bandwidth~ 0.03 cm-l FWHM) and frequency doubled in KDP. The DUMP pulse, which stimulates emission down to a rovibrational level of the ground state, would be supplied by a second similar dye laser system pumped by the same Nd:YAG laser. The optics would be arranged so that the DUMP pulse reaches the sample cell ~15 nsec after the DUMP pulse. beams. The PUMP beam is split. into two equal intensity One beam would be combined with the DUMP beam and directed into a cell containing ~100 mtorr of formaldehyde and an optical quences gas such as He. The other beam would be directed into a second half of the cell . The difference in -255fluorescence signals from both beams would be used to monitor the SEP process . The population of the nitially prepared rovibrational state or other rotational levels of the same vibrational state would be monitored by a LIF technique 20 - 500 nsec after the SEP process. A second Nd:YAG laser pumped dye laser would excite molecules in the state of interest up to a level in the A1A2 level. Fluorescence from this level would be proportional to the population of the state of interest . The fluorescence would be dispersed by a high resolution monochrometer to eliminate scattered light and fluorescence - from the level of the A state used in the SEP process. The fluorescence signal from the LIF process is expected 13 to be easily detect~ble, since Reisner and co-workers esti mated that their SEP process in formaldehyde transferred 0.02% of the total thermal population into a specific rovibrational level. Similar LIF techniques have been used in . 11 12 other SEP exper1ments. ' The possibilities for states to prepare and probe are endless. One example _of an excitation and detection scheme will no be given which could serve as an ideal first experiment. The transitions and their energies are derived from Refs. 13 and 17. The PUMP pulse at "'354 nm excites the 4~ PQ 1 3 (4) line as in Ref. 13. ' The DUMP pulses stimulates emission down to the 4 4 41 3 rovibrational state via the ' 4! PQ 1 , 3 (4) transition at 422 nm . The PROBE pulse would -256detect the population of the 4 4 4 1 , 3 level by exciting it by 24,994.6 em - 1 to a level which fluoresces at 29,621.5 em- 1 . The 31 3 and s 1 5 levels of 4 4 could also be detected by , ' exciting with 25,005.3 cm-l or 24,983.9 cm-l pulses, respectively. The same fluorescence would be observed. By varying the time delay between the SEP pulses and the PROBE pulse, the decrease in the 4 4 41 3 lev-el population and the ' increase in the 4 4 31 3 and 4 4 s1 5 populations could be ob' ' served. Many other wavelengths for PUMP, DUMP and PROBE pulses can be obtained from the spectroscopic transitions listed in Refs. 13 and 17, allowing a large variety of rotational levels to be prepared or detected. Additional transitions can be generated from spectroscopic constants of the various vibrational levels using a program by Birss. 18 This program calculates asymmetric rotor energy levels from spectroscopic constants. Several types of rotational relaxation could be studied. The experiment-to-measure self-relaxation of formaldehyde is -1 complicated because of the large concentration of A A2 states remaining after the SEP process. A better experiment would be to measure relaxation caused by collisions with helium, other rare gases or other molecules. In these experiments, a five-motor sample of formaldehyde and 95-motor sample of the collision partner would be used. Helium is especially attractive because extensive theoretical studies of -257- He/formaldehyde relaxation have been performed. 6 ' 7 Propensity rules for rotational inelastic collisions have been proposed. 6 These rules are obeyed in the theoretical He/formaldehyde results of Green et al., 7 but they have not been verified experimentally. A comparison of experimental results for this system with theory should provide a check on the reliability of the approximations used in the calculations. -258 References ~' 1. J. P.Toennies, Ann. Rev. Phys. Chern . 2. T. Oka, J. Chern. Phys. 49, 3135 (1968). 3. U. Borkenhagen, H. Ma1than and J . Peter Toennies, J. Chern. Phys. 4. 2!, 1722 (1979). M. Faubel, K. H. Koh. and J. P. Toennies, J. Chern. Phys. 5. 225 (1976). Zl' 2506 (1980). P. Anderson, H. Joswig, H. Pauly and R. Schinke, J. Chern. Phys. zz, 2204 (1982). 7. 1£, 3637,5974 (1982). S. Green, B. J. Garrison, W. A. Lester, Jr. and w. M. Miller, Astrophys. J. Supple. Series l!, 321 (1978). 8. R. T. Pack, J. Chern. Phys. 04, 1659 (1975). 9. A. T. Amos and R. J. Crispin, in Theoretical Chemistry, 6. M. H. Alexander, J. Chern. Phys. Advances and Perspectives, edit. by Eyring and D. Henderson (Academic, New York, 1976), Vol. 2, pp. 1-66. 10. C. Kittrell, E. Abramson, J. L. Kinsey, S. A. McDonald, D. E. Reisner, R. W. Field and D. H. Katayama, J. Chern. Phys . .zl, 2056 (1981). 11. W. D. Lawrence and A. E. W. Knight, J. Chem. Phys. 1£, 5637 (1982). 12. W. D. Lawrence and A. E. W. Knight, J. Chem. Phys. zz, 570 (1982). 13. D. E. Reisner, P. H. Vaccaro, C. Kittrell, R. W. Field, J. L. Kinsey and H.-L. Dai, J. Chem. Phys. zz, 573 (.1982) . -25914 . D. J . Moll, G. R. Parker, Jr., and A. Kuppermann, "Photoacoustic Detection of Stimulated Emiss i on Pumping in p-Difluorobenzene". To be published (see Paper II of this thesis). 15. V. A. Job, V. Sethuraman and K. K. Innes, J . Molecular Spectrosc. ~' 365 (1969); V. Sethuraman, V. A. Job and K. K. Innes 33, 189 (1970). 16 . K. Shibuya, R. A. Harger and E. K. C. Lee , J. Chern . Phys. ~' 17. 751 (1978). J. L. Hardwick and S. M. Till, J. Chern . Phys. ~' 2340 (1979). 18. Referred to in F. W. Birss, R. Y. Dong and D. A. Ramsay, Chem . Phys. Lett. 18, 11 (1973). -260 - Proposition III: An Experimental Investigation of the Mechanism of Plasma Formation during Metal Processing with High Power UltraViolet Lasers. Abstract: It is proposed that the mechanism of plasma formation produced by the interaction of a high power UV laser pulse with the surface of aluminum in vacuum be investigated. Impulse coupling, thermal coupling, total reflectivity, electron production and plasma threshold will be monitored as a function of wavelength and pulse energy. The appearance of two-photon resonant ionization processes should lower the plasma ignition threshold. -261- The use of lasers by the manufacturing industry has increased tremendously in the past few years. Over 1000 lasers are used daily on U.S. manufacturing lines. 1 A majority of these lasers are used for industrial welding and machining. Solid state and co 2 lasers are used in most of these applications. Typical output powers of these lasers is around 500 watts CW, but some range as high as 25 kilowatts. 2 The industrial metalworking applications of high powered lasers has produced a strong interest in understanding the dynamics of the interaction of intense light pulses with metal surfaces. anism for A general picture of the interaction mech- co 2 laser radiation at 10.6 microns is now emerging. At low laser intensities, the thermal energy deposited in the metal surface is determined by the intrinsic absorptivity of the metal. 3 For most metals, the infrared absorptivity is quite low (<5%). At higher intensities (~10 6 W/cm 2 ), the absorptivity changes abruptly due to the presence of plasma effects. In air typical plasma ~10 9 w;cm 2 • ignition intensities are At a rough metal surface, however, the local heating of surface defects causes thermionic emission of electrons. These electrons are accelerated by inverse bremsstahlung absorption of laser radiation. 6 The electrons are cascade multiplied during collisions with metal vapor atoms or air molecules. If the energy gained by the electrons is larger than that lost through collisions, a plasma can be ignited. 5 ' 6 -262The metal surface lowers the threshold intensity for plasma ignition in air by a factor of 10 3 • In CW operation , the plasma blocks the absorption of laser energy by the metal. 7 For pulsed operation, the plasma is confined to the beam interaction region during the laser pulse. It absorbs the laser energy and reradiates it in the UV or vacuum UV where the metal has a strong absorption . This produces an efficient transfer of energy to the metal surface, even more efficiently than direct infrared absorption. 8 ' 9 As the laser intensity is increased, the energy per unit area deposited . into the surface remains essent1ally constant. 10 ' 11 The plasma pressure is increased by absorption of more energy, expands faster and deposits energy into a larger surface area. This spreading process prevents the actual melting of the metal surface in many cases because the laser energy cannot be localized .within the laser spot size . 6 When the volume surrounding the metal sample is evacuated, the interaction mechanism changes slightly . The air which previously supported the plasma is no longer present. Instead, small metal surface defects are vaporized and 12 ionized. 6 ' 7 This mechanism was verified by Walter et a1. who observed emission from ionized and excited metal atoms simultaneously with the increased absorption by the plasma . Several theoretical models of ionization after metal vaporization have been developed. 13 , 14 The vacuum case is peculiar in that the plasma does not propagate outward significantly -263from the focal spot with increasing intensity. 6 is not fully understood. This result Metal surface melting is achieved easily in the vacuum metal vapor plasma case because the energy remains localized within the focal spot . There appears to be no upper limit to the usable laser intensity for applications involving sample surfaces in vacuum. Only recently have high power UV lasers become available for experiments to determine the mechanism of ultraviolet laser pulse interactions with metal surfaces. 15-18 Duzy and . d.1ate d an a 1 um1num . . h a co-wor k ers 16 1rra sur f ace un d er vacuum w1t XeF excimerlaser at 353 nm. They monitored the mechanical impulse imparted to the sample by the action of the laser pulse, and observed a low fluence threshold c~z J/cm 2 ) for the appearance of this impulse. Bulk vaporization and ejection of aluminum from the surface is not expected since that requires a fluence of ~40 J/cm 2 • The inverse bremsstrahlung absorption is proportional to A2 • Fluences greater than 100 2 . J/cm would be necessary to produce a plasma by that mechanism. The impulse coupling coefficient was two to four times larger than observed in similar IR experiments. Similar overall 15 effects were observed for KrF laser excitation at 249 nm. Rosen and co-workers 17 performed similar experiments on aluminum surfaces. They modeled the interaction by including two significant mechanisms for laser-vapor interaction: (1) the high temperatures at the surface could produce vaporized aluminum atoms in excited states which could be ~264- ionized by absorption of one laser photon, and (2) inverse bremsstrahlung increases with the density of the vapor and with the percentage of ionization in the vapor. The experi- mental results on impulse coupling and plasma ignition threshold were in good agreement with their theoretical model predictions. Increased deposition of energy into the surface was observed above the plasma ignition fluence threshold of 30 J/cm 2 • The model of Rosen et a1~ 7 did not consider the possi- bility of two-photon ionization processes in atomic aluminum vapor. The high intensities employed in these experiments (~60 megawatta/cm 2 ) and the relatively low ionization po- tential of aluminum (5.984 eV) 19 make the probability of a two-photon nonresonant ionization process at XeF laser wavelengths very high. In this proposition, it is proposed to investigate experimentally the role of two-photon ionization processes in the interaction of intense UV laser pulses with metal surfaces. The experiments would involve focusing the output of a tunable high power pulsed UV laser onto a 2024-T3 aluminum surface in vacuum. The wavelength would be scanned through a strong resonance between a low-lying and intermediate level of aluminum atoms while monitoring the following experimental parameters: (1) impulse coupling, (2) thermal coupling, (3) total reflectivity, (4) electron production, ·and (5) plasma threshold. The laser system and experimental techniques will be described in the following section. -265 Laser System. The experiments of Duzy et a1 . 16 used a XeF excimer laser with an output of up to 7 J at 353 nm with ~1 ~sec pulsewidths. Tunable UV lasers are not capable of equaling this performance . However, by using slightly tighter focusing, the same fluence and intensities can be produced. The highest power tunable UV laser in the 353 nm region currently available is a frequency doubled alexandrite laser. 20 The fundamental tuning range of this laser is 700815 nm. Pulse energies of 3.5 J have been produced. The Q-switched pulsewidth is variable from 30 to 600 nsec. Fre- quency doubling in KDP crystals of pulses with these characteristics can easily have conversion efficiencies of up to 40%. Therefore, this laser should be capable of producing UV pulses tunable from ' 350 to 407 nm with a maximum pulse energy of 1.5 J. This is within a factor of 2 of the typical . 16 17 pulse energies used in the fixed wavelength exper1ments. ' Therefore, the detection techniques employed in other surface interaction experiments should be usable in this experiment. Aluminum atoms have a strong absorption from the ground state at 394.401 nm 19 which corresponds to the 3s 2 3p-3s 2 ('S)4s transition array. The oscillator strength f of this trans- ition is 0.115. This is the only transition from the ground state in this wavelength region. Two other hot band ab - sorptions could occur at 395.57 and 396.152 nm. When the laser is in resonance with the transition at 394 . 401 nm, the rate of production of ions and electrons from vaporized -266· aluminum should increase by several orders of magnitude. This should increase the impulse and thermal coupling due to plasma formation, and lower the threshold for plasma ignition. The untreated aluminum surface absorptivity ranges from a few percent at IR wavelength to ~20% at 350 16 nm, and increases to ~40% at 249 nm. 15 This increase in absorptivity is gradual compared to the sharp absorption line of aluminum atoms at 394.401 nm. The effects caused by the atomic absorption should be easily distinguishable from the metal surface absorptivity. Impulse Coupling. The coupling of a mechanical impulse to a metal sample can be produced by (1) expulsion of metal liquid, vapor or fragments from the surface, or (2) pressure on the surface from the creation of a plasma which creates a . 17 21 laser supported detonat1on wave (LSD). ' The photon momentum transfer is insignificant even at the high intensities of these experiments. As the fluence of the incident pulse is increased, impulse coupling is first detected when vaporization occurs. The impulse may increase further when a plasma is ignited. Impulse coupling would be measured as a function of laser wavelength and intensity using the pendulum method described by Rosen et a1. 17 A small aluminum sample is attached to a pendulum of length 1 of ~20 em and effective mass m of ~s g. An optical projection system with a magn~fication ratio M would small deflections X of the be used to monitor pendulum after a laser shot. -267The impulse I is given by I = m(X/M)/g,li where g is the atceleration due to gravity. Thermal Coupling. The efficiency of .heat deposition into the sample would be monitored with a chromel-alumel thermocouple mounted in a small hole drilled into the back of the target. 22 The thermal coupling as a function of increasing intensity may first increase, then decrease at the onset of vaporization and increase again at plasma ignition. These effects would be monitored as a function of laser wavelength. Total Reflectivity. As a plasma is ignited, the inverse Bremsstrahlung process, ionization processes and plasma absorption cause a large decrease in total reflectivity. 23 The total reflectivity is monitored by placing the sample at one focal point of a rotational ellipsoid. The reflected laser light is collected at the other focal point and detected with a fast photodiode. A narrow pass filter centered on the wave- length region of interest would prevent broadband plasma emission from being detected. A second identical photodiode would monitor the temporal characteristics of the laser pulse. By comparing the signals from the two photodiodes, the exact time of plasma formation during the pulse could be monitored as a function 6f laser wavelength. Electron Production. The production of electrons and ions, as a function of incident pulse intensity and wavelength, -268- could be measured easily by applying a bias of a few hundred volts between the metal surface and a nearby electrode, and measuring the current which flows after each laser pulse. A charge sensitive preamplifier would integrate the current pulse as described by Rianda et a1. 24 The production of electrons as a function of wavelength should indicate if resonant two-photon ionization of aluminum is occurring at 394.401 nm. Plasma Threshold. The appearance of a plasma is in- directly detected by the previous four techniques. However, a more direct way of determining the plasma ignition threshold is to observe the broadband emission from the plasma. This emission could be detected photographically or with a fast photodiode. A narrow band rejection filter wduld be used to exclude scattered light from this incident laser pulse. The five techniques just mentioned should provide a fairly complete approach to studying the interaction of an intense UV pulse with an aluminum surface. In particular, it is hoped that many of the techniques will reveal enhanced plasma effects when the laser is tuned to 394.401 nm. At that wavelength, ground state aluminum can be ionized efficiently by a two-photon resonant process. The production of a plasma is important for many laser machining processes. Enhanced thermal coupling and melting of metal can occur when a plasma is present. The plasma pressure also can effectively force molten metal out of the -269- holes produced in laser drilling or machining processes. 25 ,Z 6 , 23 If resonant two-photon ionization occurs, the plasma ignition threshold may appear at lower intensities and fluences, allowing the use of much smaller lasers for a variety of metalworking applications. -2 7 0- References l. "Industrial Lasers in the U.S.: Vital Statistics", Laser Focus, Sept. 1982, p. 38. 2. S. L. Ream, Laser Focus, Dec. 1982, p. 43. 3. R. G. Root, J. Phys. Colloq. (France), 3rd Int. Symposium on Gas Flow and Chern. Lasers 4l(C-9), C9-59-73 (1980). 4. A. I. Barchukov, F. V. Bunkin, V. I. Konov and A. M. Prokhorov, Sov. Phys. JETP Lett. 5. !I, 294 (1973). A. I. Barchukov, F. V. Bunkin, V. I. Konov and A. A. Lyubin, Sov. Phys. JETP ~, 469 (1974). 6. J. A. McKay and J. T. Schriempf, IEEE J. Quant. Elect. QE-17, 2008 (1981). 7. C. T. Walters, R. H. Barnes and R. E. Beverly III, J. Appl. Phys. 8. ~, 2937 (1978). A. N. Pirri, R. G. Root and P. K. S. Wu, AIAA J. 16, 1296 (1978). 9. I. V. Nemchinov, I. A. Polozova, V. V. Svettsov and V. V. Shuvalov, Sov. J. Quant. Electron. 10. 2, 721 (1979). J. A. McKay and J. T. Schriempf, Appl. Phys. Lett. 31, 369 (1977). 11. J. A. McKay, R. D. Bleach, D. J. Nagel, J. T. Schriempf, R. B. Hall, C. R. Pond and S. K. Manlief, J. Appl. Phys. ~, 12. 3231 (1979). W. T. Walter, N. Solimene, K. Park, T. H. Kim and K. Mukherjee, "Optical Properties of Metal Surfaces During Laser Irradiation," Lasers in Metallurgy, AIME-TMS Conf. Series. -27113. R. J. Harrach, J. Appl. Phys. 48, 2370 (1977). 14. N. Newstein and N. Solimene, IEEE J. Quant. Elect. QE-17, 2085 (1981). 15. J. A. Woodroffe, J. Hsia and A. Ballantyne, Appl. Phys. Lett. 36, 14 (1980). 16. C. Duzy, J. A. Woodroffe, J. C. Hsia and A. Ballantyne, Phys. Lett. lr, 542 (1980). Appl. 17. D. I. Rosen, J. Mitteldorf, G. Kothandaraman, A. N. Pirri and E. R. Pugh, J. Appl. Phys. 18. 3190 (198). D. I. Rosen, D. E. Hastings and G. M. Weyl, J. Appl. Phys. 19. ~, ~' 5882 (1982). W. L. Wiese, M. W. Smith and B. M. Glennon, Atomic Transition Probabilities, Vol. II, NSRDS-NBS 22, SD Cat. # CB.48:22 20. (1969). Allied Chemical Electro-Optical Products brochure, "Only Alexandrite Offers All These Features", Box 1021R, Morristown, N.J. 07960. 21. R. D. Dupuis and P. D. Dapkus, Appl. Phys. Lett. 32, 473 (1978). 22. J. A. McKay, J. T. Schriempf, T. L. Cronburg, J. E. Eninger and J.A. Woodroffe, Appl. Phys. Lett. ~' 125 (1980). 23. M. VonAllmen, P. Blaser, K. Affolter and E. StUrmer, IEEE J. Quant. Elect. QE-14, 85 (1978). 24. R. Rianda, D. J. Moll and A. Kuppermann, Chern. Phys. Lett. Zl, 469 (1980). -27225. J. Fox, Appl . Phys. Lett. 26, 682 (1975). 26. V. , Shui; Phys. Fluids ~, 2174 (1978). -273- Proposition IV: H2o Vapor Detection in the Chemical Oxygen Iodine Laser by a New Dye Laser Intracavity Absorption Technique. Abstract: It is proposed to use a new dye laser intracavity absorption technique to monitor the H20 vapor concentration in the Chemical Oxygen Iodine Laser. A small change in wavelength makes this techique also applicable to ground state o2 detection. -274- The work of Derwent and Thrush on energy transfer between 0 2 and r 2 1 - 4 formed the basis for discovery of the chemical generation of a population inversion between the spin-orbit states of atomic iodine. 5 Lasing action was ob- served using the o2 -r 2 system by McDermott and co-workers. 6 A short time later, this laser was scaled up to produce 100 watts of CW output. 7 This laser has the potential of being scaled up to produce output powers comparable to the current DF military lasers. 8 The chemical oxygen iodine laser (COIL) operates by mixing excited state o 2 c 1 ~g) with I 2 • is believed to occur. 6 The following mechanism o;ca 1 ~g) + o;ca 1 ~g) ~ o;*co 1 ~;) + o 2 cx 3 ~;) (1) o;*co 1 ~;) (2) * 1 02(a ~g) I * ( 2P 1; 2 ) + I 2 ~ O(X 3 ~;) + 2I( 2P 2 -3 I( p3/2) ~ 02(X ~g) + ~ I( 2P 3; 2 ) + hv(A + 312 ) * 2 I ( pl/2) = 1.315 ~m) (3) l4) The o*2 (a 1 ~g) can be generated chemically in a variety of ways. One method currently under consideration by TRW Defense and Space Systems Group involves sparging Cl 2 gas through NaOH in liquid H o . 8 The stoichiometry of the reaction is 2 2 C1 2 + 2H 2o + NaOH ~ 3 0 *2 + NaOH + H20 + 2HC1 . 2 z (S) The 0 *2 is extracted from the sparger and injected into a laser cavity simultaneously with I 2 , which initiates lasing -275- action. The collisional deactivation of o; is very slow, 9 allowing efficient transfer of o* 2 from the sparger to the laser cavity. Unfortunately, the H2o produced in the sparger is an 5 extremely efficient deactivator of 0 ** 2 , almost 10 times H20 also deactivates o*2 and I*, but with a lower efficiency. 11 , 12 is necessary to more efficient than o2 • 10,11 o;* dissociate I 2 (Eq. 2). 13 The deactivation of 0 ** 2 should significantly decrease the performance of the COIL. The addition of a cold trap between the sparger and laser cavity reduces the H2o;o 2 ratio from 2 to ~o.l. 8 High efficiency cold traps may be able to eventually reduce this ratio to 0.0001. 8 The preliminary experiments needed to scale up this laser require a fast, accurate, real-time technique for measuring the H20 concentration between the cold trap and laser cavity. It is proposed to use a new technique, photo- acoustic detection of intracavity absorption, to measure this H2o concentration. Intracavity absorption has been widely used to detect . 14-24 Gas phase absorption coextremely weak absorpt1ons. efficients as small as 10- 8 cm-l 20 · and 10 -9 em -1 17 have been measured. Using this technique involves placing the sample of interest inside the cavity of a CW dye laser and observing the change in the laser's output wavelengths or power. Two different categories of measurements are possible. -276In case (a), the laser mode falls within the absorption linewidth of the sample. In case (b), some or many of the lasing modes fall outside the absorption linewidth. When a sample is inserted into a cavity under case (a) conditions, the resulting increased cavity loss produces a drop in laser power. In case (b), the absorption partially quenches only the modes which fall within the absorption linewidth. power in these modes is shifted to other modes. The The total output power under these conditions is not an accurate indicator of the absorption. Instead, a frequency sensitive detector must be used to monitor the redistribution of output power among the different modes. Measurements made under case (a) conditions have enhancements which are largely the result of the increased effective optical path through the sample. 25 Under case (b) conditions, however, much larger enhancements of up ~o 10 5 have been observed. 15 Three techniques have been used to observe the redistribution of output frequencies under case (b) conditions. (1) The spectrum of the laser output is taken using a high resolution spectrometer . 16 ' 21 ' 22 (2) A small amount of identical sample is placed in a fluorescence cell outside the cavity. 15 The laser output produces fluorescence from this cell, which decreases when the sample is placed in the cavity. Since the sample in the fluorescence cell absorbs only the lines which are quenched by the intracavity sample, - 277- the redistribution of output wavelengths away from the quenched lines is monitored. (3) The fluorescence cell of technique (2) is replaced by a hollow cathode discharge lamp containing the same material as the sample. The opto - galvanic effect is used to monitor absorption in the hollow cathode lamp. 24 Techniques (2) and (3) provide fast, accurate and economical methods of monitoring the redistribution of output in multi-mode lasers when a selective absorber is placed in the cavity. These techniques are limited, however, to molecules which fluoresceor atoms which exhibit an optogalvanic effict. The photoacoustic detection technique provides an alternative technique. Since it responds to nonradiative collisional decay pathways, it works well for states of molecules with low quantum yields for fluorescence . In the proposed H20 detection scheme for the COIL, the diagnostic duct between the cold trap and cavity would be equipped with two heated windows and placed inside the cavity of a broadband CW dye laser pumped by a mechanically chopped ion laser . The output of the dye laser would be directed into a temperature stabilized photoacoustic cell containing a H20 vapor sample. The photoacoustic signal would be processed by a lock-in amplifier referenced to the mechanical chopping frequency. In this configuration, the decrease in photoacoustic signal would be proportional to the amount of H2o in the diagnostics duct . -278 Several H20 absorption lines could be used; however, the presence of Cl 2 , HCl, o2 , o*2 and o** 2 requires a careful choice to avoid overlapping absorption bands. 26 One possible choice would be the H20 overtone bands between 715 and 732 nm . One of the strongest H2o lines in this region is the 303 + 404 (J,Ka,Kc) rotational line of the 301 (v 1 ,v 2 ,v 3 ) vibrational transition at 719~1488 nrn. 27 The peak absorption cross section cr 1 of this line is relatively insensitive to temperature changes. 27 cr 1 can be calculated from the linestrength S of 40.1 x lo- 24 em -1 /(molec . /cm 2 ) using the formula crL = S/TiyL (6) . 27 where y 1 is the half-width at half max1mum. The total pressure in the diagnostic duct is typically 1 torr. 8 At this pressure, Doppler broadening should contribute almost At 298°K the H2o half-width in this wavelength region due to Doppler broadening 28 is about 0.02 cm- 1 , yielding a peak absorption cross section of 6.4 x lo- 22 all of the linewidth. em 2 • This linewidth easily allows case (b) intracavity ab- sorption measurements using a broadband dye laser with a simple birefringent tuning element. Using this cross section 17 and assuming a minimum detectable absorption coefficient of 10 -9 ern -1 , the photoacoustic intracavity absorption technique should be able to measure H20 partial pressures down to 5 x 10~ 5 torr. This is a factor of 2 better than·the lowest expected H20 partial pressure. - 279 This H20 absorption line falls near the peak of the gain curve for oxazine 725 laser dye when pumped by a kryton ion laser . 29 The oxygen X3 rg- + b 1·rg+ (0,0) transition also falls on this dye gain curve . By simply changing wave - lengths and having both 0 2 and H20 in the photoacoustic cell the o2 concentration could also be measured . The 0 pp 2 q 30 transition at 763 . 6328 nm has a linestrength of 7 . 44 x lo- 24 em -1 /(molec./cm 2 ). 31 Repeating the calculation for H20 for the o2 case gives a minimum detectable 0 2 partial pressure of 2 x 10- 4 torr. The new photoacoustic detection of intracavity absorption technique just described should provide a fast, accurate, realtime method for monitoring the H2 o and o2 partial pressures in COIL systems. The expected sensitivity for H20 should easily monitor the minimum projected concentrations when more advanced cold traps are employed. When combined with the existing dimol emission technique for monitoring concentrations, 32 o; the three highest concentration species, H2o, o2 and o*2 , present in the cold trap output can be monitored . This should greatly simplify the scaling up of this promising laser system. - 280References 1. R. G. Derwent, D. R. Kearns and B. A. Thrush, Chern. Phys. Lett.£, 115 (1970). 2. R. G. Derwent and B. A. Thrush, Chern. Phys. Lett. ~' 591 (1971). 3. R. G. Derwent and B. A. Thrush, J. Chem. Soc . Faraday Disc. 4. II~' 720 (1972). R. G. Derwent and B. A. Thrush, Disc. Faraday Soc. il' 162 (1972). 5. A. T. Pritt, Jr., R. D. Coombe and D. Pilipovich, Appl. Phys. Lett. 31, 745 (1977). 6. W. E. McDermott, N. R. Pchelkin, D. J. Benard and R. R. Bousek, Appl. Phys. Lett. 32, 469 (1978). 7. D. J. Benard, W. E. McDermott, N. R. Pchelkin and R. R. Bousek, Appl. Phys. Lett. 8. l!, 40 (1979). Technical Handout for AE107, a course on High Energy Chemical Lasers taught at Caltech by TRW Defense and Space Systems Group. 9. Fall quarter, 1980/81. K. H. Becker, W. Groth and U. Schurath, Chem. Phys. Lett. !, 259 (1971). 10 . E. A. Ogryzlo and B. A. Thrush, Chern. Phys. Lett. ~' 314 (1974). 11 . H. H. Wasserman and R. W. Murray, Singlet Oxygen, Organic Chemistry, A Series of Monographs, Vol . 40 (Academic Press, New York, 1979), pp. 43-48. -28112. A. J. Grimley and P. L. Houston, J. Chern. Phys. 69, 2339 (1978). 13. There is some recent but undocumented evidence from Ref. 8 that I 2 may be dissociated by a second mechanism which operates in parallel with reaction (2). 14. N. G. Peterson, M. J. Kurylo, W. Braun, A. M. Bass and R. A. Keller, J. Opt. Soc. of Amer. 61, 746 (1971). 15. T. W. Hansch, A. L. Schawlow and P. E. Toschek, IEEEE J. Quant. Elect. 16. ~' 802 (1972). J. P. Reilly and G. C. Pimentel, Appl. Opt. 15, 2372 (1976). 17. V. M. Baev, T. P. Belikova, E. A. Sviridenkov and A. F. Suchkov, Sov. Phys. JETP 47, 21 (1978). 18. V. R. Mironenko and I. Pack, Sov. J. Quant. Elect. ~' 1394 (1978). 19. F. J. Morgan, C. H. Dugan and A. G. Lee, Opt. Commun. ~' 20. 451 (1978). W. T. Hill III, R. A. Abreu, T. W. Hansch and A. L. Schawlow, Opt. Commun. ~' 96 (1980). 21. S. J. Harris and A.M. Weiner, Opt. Lett. i, 142 (1981). 22. S. J. Harris and A. M. Weiner, J. Chern. Phys. 74, 3673 (1981). 23. Wm. R. Lambert, P. M. Felker and A. H. Zewail, J. Chern. Phys. 24. 2!, 4732. E. F. Zalewski, R. A. Keller and C. T. Apel, Appl. Opt. ~' 1584 (1981). -28225. H. K. Holt, Phys. Rev. All, 625 (1975). 26. The r 2 concentration is expected to be low in the diagnostic duct because the r 2 is added in the laser cavity. The near supersonic flows required to sustain high power ~asing prevent backflow . 8 27. T. D. Wilkerson, G. Schwemmer, B. Gentry and L. P. Giver, J. Quant. Spectrosc. Radiat. Transfer 22, 315 (1979). 28. A. Yariv, Quantum Electronics, 2nd edit. (John Wiley, New York, 1975), p. 167. 29. Laser Dye Brochure, Exciton, P.O . Box 3204, Overlook Station, Dayton, OH 45431. 30. H. D. Babcock and L. Herzberg, Astrophysical J. 108, 167 (1948). 31. J. H. Miller, R. W. Boese and L. P. Giver, J. Quant. Spectrosc. Radiat. 32 . Transfer~' 1507 (1969). R. G. Derwent and B. A. Thrush, Trans. Faraday Soc. 67, 2036 (1971). - 283- Proposition V: Abstract: A Radio-Chemical Tracer Study of the Tungsten Regenerative Cycle in Incandescent Lamps Containing CBrF 3 • It is proposed that the migration of tungsten in incandescent lamps containing CBrF 3 be studied by a tungstenISS radio-chemical tracer technique. The direction of mi- gration of tungsten-185 depends on whether bromine or fluorine is involved in the regenerative cycle. A direct measurement of this migration should help elucidate the mechanism of regeneration. -284~ The lifetime of conventional gas-filled tungsten incandescent filament lamps has been shown to be inversely proportional to the rate of evaporation of tungsten from the filament. 1 After extended operation, a local "hot spot" develops on the filament. This "hot spot" is located at a position where extra tungsten has evaporated off, leaving behind a smaller cross sectional area with a higher resistance.2 The increased resistance elevates the temperature of the filament at that point, which increases the rate of evaporation. When the lamp is switched on, a nonequilibrium temperature overshoot occurs at the "hot spot". 3 ' 4 If the overshoot temperature exceeds the melting point of tungsten (3410°C), · the filament fuses at the "hot spot" and the lamp is destroyed. Tungsten atoms or molecules which evaporate from the incandescent filament surface may either transfer to nearby parts of the filament, or escape the high-temperature lowviscosity Langmuir sheath 4 surrounding the filament and diffuse out to the glass envelope where they form a dark metallic film. The accumulation of tungsten on the walls places a limit on the minimum dimensions of the lamp. The addition of small amounts of halogens to the lamp produces a regenerative cycle which transfers the tungsten back to the filament. 6 In the simplest mechanism, an equilibrium is established between tungsten metal and tungsten halogen compounds. 7 ' 8 -285 - W + nX,. WX (1) n At low temperatures, the equilibrium favors WXn, while at high temperatures dissociation occurs to release tungsten metal. Physically, this causes WXn compounds to form in the cooler envelope regions. WXn diffuses back into the hot filament region, decomposes and returns the tungsten to the filament. The halogen cycle continually cleans the envelope walls. This allows the lamp size to be decreased, making it economically feasible to add high pressures of krypton or xenon instead of the usual lighter argon. When quartz envelopes are used, krypton or xenon pressures of up to 10 atmospheres can be used. The high pressures and heavier gases reduce tungsten diffusion away from the filament, allowing operation at higher intensities and up to 100% longer lifetimes. 6 Typical quartz halogen lamps use iodine, bromine or chlorine halogen cycles. Unfortunately, the tungsten halides formed from these halogens dissociate at temperatures well below the normal filament operating temperature . 9 The tungsten halides which are returning from the walls dissociate in the cooler filament regions, before they are able to reach the central hottest part of the filament. The typical halogen cycle, therefore, eliminates the problem of radial diffusion of tungsten to the envelope walls, but -286- enhances the axial flow of tungsten from hotter filament areas to cooler areas of the filament. The current quartz halogen lamp technology has greatly increased the brightness and slightly increased the lamp lifetime. The problem of "hot spots", however, still remains as the major cause of lamp burnout. The axial transport of tungsten can be minimized by using fluorine in the halogen cycle. 10 Tungsten fluoride (WF 6 ) is stable up to ~3100°C. WF 6 , therefore, dissociates only at the hottest filament surfaces. This process replaces the evaporated tungsten preferentially at the thinnest filament areas which are the "hot spots" where filament failure occurs. Schroder was the first to demonstrate this "self-healing" fluorine cycle process. 10 11 ' Even with the demonstration of self-healing fluorine lamps, the fluorine·cycle has still not appeared in commercial lamps because of several additional problems. First, there is a large temperature gradient along the filament axis. Tungsten near the cool ends of the filament reacts with the fluorine, and gets transported to the hotter center area of the filament. This process is called tail erosion. 6 A second problem arises from the reactivity of fluorine com pounds with the quartz envelope. In particular, the following reaction with WF 6 is believed to occur. 6 (2) -287- The oxyfluorides then dissociate to give tungsten oxides, which deposit on the enyelope. Reaction (2) rapidly consumes the fluorine in the lamp, destroying the regenerative fluorine cycle. Various envelope coatings such as MgF 2 and CaF 2 , 12 13 and glass films based on Al 20 3 -Ti0 2 -P 2o5 have been tried in attempts to protect the quartz surface. The problems inherent in the fluorine cycle have prevented a widespread commercial application of this technique. Recently, Hill and Dolenga 14 reported that addition of small amounts of CBrF 3 to the fill gas extended the lamp lifetime by a factor of two over lamps containing bromine alone. The normal lamp manufacturing processes which removed trace impurities of oxygen were omitted in their lamps. They claimed that small amounts of oxygen were necessary to form the dominant transfer cycle molecule wo 2F 2 • Previous calculations by Neumann 15 had predicted that wo 2F 2 would be the dominant specie at temperatures below 2S00°K. Lamps containing 6 atm of krypton and 100 ppm CBrF 3 which were operated at a filament temperature of 3250°K exhibited lifetimes of ~350 hours. Similar lamps which contained CH 2Br 2 had life- times which were ~140 hours. The fluorine did not appear to attack the lamp walls or the cooler part of the filament. The results of Hill and Dolenga have been criticized strongly by Coaton and Fitzpatrick 6 , who suggest that the CBrF 3 present may thermally decompose to BrF 3 . This compound -288is stable up to 2000°K, which produces no free fluorine at the walls or filament tails. They postulate that a fluorine cycle does not occur in these lamps. Instead, the bromine is responsible for the regenerative mechanism which keeps the walls clean. I propose the use of a radio-chemical tracer study of the CBrF 3 lamp to determine whether a completely regenerative fluorine cycle is operating, or if only a radially regenerative bromine cycle is occurring. Using the technique of Bartha and Hangos,16· a small sample of W-185 radioactive isotope would be added to the center of a tungsten filament. This is accomplished by preparing a solution of ammonium 17 tungstate containing W-185. One drop of solution is placed on the filament and allowed to dry. A hydrogen atmosphere would reduce the tungsten to tungsten metal. Lamps prepared . using the method of Hill and Dolenga 14 containing CBrF 3 and the W-185 labeled filament would be burned for a short time. Then the location of the W-185 would be monitored. W-185 decays with a half-life of 75.8 . . · 1 e. 18 The W-185 d ays by emlss1on o f a 0 • 432 Me V b eta part1c location on the filament or envelope would be detected with a silicon surface-barrier detector 19 placed behind an aperture in a lead plate. By moving the filament slowly across the aperture and detecting the emitted beta particles, the relative concentration of W-185 in different sections of the filament would be determined. -289- Two different distributions of W-185 could be expected depending on whether a fluorine or bromine cycle is operating. The bromine cycle removes tungsten from the hot central portion of the filament and deposits it on the cooler ends. Since the W-185 would be initially placed in the center, the bromine cycle would slowly move the W-185 out to the ends of the filament . The fluorine cycle, if operating correctly, is a self-healing process which deposits evaporated tungsten back on the central hot part of the filament. Therefore, the W-185 should remain localized in the center of the filament and not appear at the ends if a fluorine cycle is operating. This radio-chemical tracer technique should be a sensitive and direct probe of tungsten migration in CBrF 3 lamps. The ability to differentiate between the bromine or fluorine cycles in these lamps should greatly increase our understanding of this complex chemical behavior. -290References l, 83 (1973). 1. E. J. Covington, J. Ilium. Eng. Soc. 2. J. R. Coaton and J. M. Rees, Proc. IEE 124, 763 (1977). 3. D. H. Fax, H. G. Sell and R. Stickler, Illuminating Engineering~' 187 (1971). 1, 295 (1974). 4. F. J. Harvey, J. Ilium. Eng. Soc. 5. I. Langmuir, The Collected Works of Irving Langmuir, Vol. 2 (Pergamon Press, 1960). 6. J. R. Coaton and J. R. Fitzpatrick, IEE Proc. 127, 142 (1980). 7. T. Geszti and I. Gaal, Acta Tech. Acad. Sci. Hung. ~' 479 (1974). 8. S. K. Gupta, J. Electrochem. Soc., Solid State Sci. and Techn. 125, 2064 (1978). 9. A. Rabenau, Angew. Chern. Internat. Edit. i, 68 (1967). 10. J. Schroder, Tech. Rev. ~' 111 (1965). 11. J. Scrhoder, Philips Tech. Rev. ~' 359 (1964). 12. British Patent 1047302 (1966). 13. R. Rothon and J. M. Rees, Chern. and Industry (1 July P~ilips 1978)' p. 478. 14. J. C. Hill . and A. Dolenga, J. Appl. Phys. 48, 3089 (1977). 15. G. M. Neumann, Z. Metallkd. 64, 26 (1973). 16. L. Bartha and I. Hangos, Int. J. Appl. Radiation and Isotopes 24, 605 (1973). -29117. Tungsten-185 is commercially available from Atomic Energy of Canada, Ltd. Commercial Products; Amersham Corp.; and New England Nuclear. 18 . Handbook of Chemistry and Physics, 57th edition (CRC Press, Ohio, 1976), p. B-322. 19 . Silicon surface barrier detectors are available from Ortec Inc., 100 Midland Rd., Oak Ridge, TN 37830. The features and operation of these detectors are described in their catalogs .